Por razones éticas, es muy difícil la evaluación de los efectos de cualquier sustancia durante el periodo de embarazo en humanos, por lo que modelos que simulen algunas características de esta etapa son de relevancia en la investigación de efectos a nivel reproductivo. Debido a su permanencia continua en el medio acuático y a su alta susceptibilidad a estresores exógenos, los peces son considerados como uno de los principales biomarcadores de la salud de los ecosistemas acuáticos. El pez guppy (Poecilia reticulata) es una especie ovovivípara que se caracteriza por la procreación de crías vivas. Debido a esta peculiaridad, esta especie puede ser considerada como una alternativa importante para la evaluación de efectos de xenobióticos, muy especialmente aquellos causados por exposición durante la etapa de gestación. Datos experimentales sugieren que las sustancias capaces de alterar el sistema hormonal conocidas como Disruptores Endocrinos (DEs), son altamente dañinas para la biota acuática. En Consecuencia, la salud reproductiva de varias especies de peces puede verse afectada desfavorablemente. Múltiples evidencias también alertan sobre la alta contaminación del medio acuático por sustancias de origen farmacéutico y sus efectos sobre el Sistema Endocrino. Los medicamentos que contienen componentes hormonales merecen especial atención. Las Progestinas Sintéticas son una gran familia de hormonas derivadas de la progesterona, que son ampliamente utilizadas en la anticoncepción humana y animal, y en terapias de reemplazo hormonal. Algunos estudios han documentado los efectos de disrupción endocrina de estos compuestos. Sin embargo, los efectos de estos fármacos sobre los ecosistemas acuáticos han sido muy poco estudiados. Se evaluaron los efectos del Acetato de Medroxiprogesterona (MPA) sobre la gestación del pez guppy. Se ensayó la toxicidad aguda en siete grupos de seis peces guppy embarazadas. Se expusieron a concentraciones de MPA: 2.5 mg/L, 5 - mg/l, 10 mg/l, 50mg/l, 100 mg/L, 150 mg/l y un grupo sin tratar (grupo control). Las concentraciones fueron preparadas a partir de formulaciones inyectables de 150 mg/mL de MPA, disueltas en solución salina normal al 0.9%. Para los ensayos se siguieron los lineamientos de la OECD para pruebas de toxicidad en peces, y todas las actividades experimentales se realizaron con estricto apego a las normas de bioseguridad establecidas por el Equipo Técnico del Ministerio de Salud de Panamá (MINSA). Con respecto a la toxicidad aguda en las 96 horas establecidas para el ensayo, no se presentó letalidad en ninguno de los grupos evaluados incluyendo el control. En los grupos de 2.5 y 5 mg/L no se observaron cambios conductuales significativos, en los grupos expuestos a concentraciones mayores, se presentó nado errático a las pocas horas de la exposición y luego, una disminución casi total de la actividad motora. Estos cambios fueron dependientes de la concentración Con respecto a los efectos sobre la gestación, todos los peces del grupo control procrearon sus crías, observando el parto de dos peces después de las 48 horas (24 crias). En los siguientes 3 y 5 días, los cuatros peces restantes del grupo control, procrearon 41 crías. En este grupo nacieron 65 crías en total. Por el contrario, ninguno de los peces de los grupos expuestos al MPA parieron ninguna cría. Se observó reducción de la mancha de gravidez y del tamaño del vientre aproximadamente a los 7-10 días después de la exposición. Esto indica que la toxicidad a nivel reproductivo del MPA es alta, ya que se interrumpió totalmente la gestación a todas las concentraciones evaluadas, lo cual sugiere muerte de los productos y resorción fetal. Aunque el mecanismo de la toxicidad reproductiva no puede deducirse en este estudio, si se consideran conjuntamente los cambios en la conducta motora observados, probablemente los efectos estén asociados con alguna manifestación de estrés oxidativo a nivel de diversos órganos en la homeostasis del pez Guppy en la etapa de gestación.For ethical reasons, the evaluation of the effects of any substance during pregnancy in humans is not permitted. Thus, the models that simulate some characteristics of this stage, are relevant in the investigation of reproductive effects, in special, those that alter the endocrine system in some way. Due to their high susceptibility to exogenous stressors, fish are considered to be one of the main biomarkers of the health of aquatic ecosystems. The guppies (Poecilia reticulata), are an oviviparus species characterized by the procreation of live offspring. For this reason, this species may be an important alternative in the evaluation of effects of xenobiotic compounds, particularly those considered to be caused by exposure during pregnancy. "Endocrine Disruptors" are substances that have the ability to alter the hormonal system, and consequently, could be hazardous to the reproductive health of various fish species. Multiple lines of evidence call attention to the high contamination of the aquatic environment with substances of pharmaceutical origin and their deleterious effects on the Endocrine System. Drugs that contain hormonal components should be given particular attention. Synthetic progestins are a large family of hormones derived from progesterone, which are widely used in human and animal contraception, and in hormone replacement therapies. Some studies have documented the endocrine disrupting effects of these compounds, however, there is still scarce knowledge about the effects of progestins the aquatic biota. The effects of Medroxyprogesterone Acetate (MPA) on gestation in guppy fish were evaluated. An acute toxicity test was conducted in seven groups of six pregnant female guppy fish, which were exposed to MPA concentrations of 2.5 mg/L, 10 mg/L, 100 mg/L, 150 mg/L and an untreated group (control group). Dilutions in 0.9% normal saline solution were prepared from 150 mg/mL injectable formulations of MPA. The tests were performed following the OECD guidelines for toxicity tests in a fish. Adittionally, all experimental activities were carried out in strict adherence to the biosafety standard established by the MINSA Technical Team of Panama. No individual in the control water group, nor in the different concentrations of MPA tested groups, died during the 96 houres after exposure in the acute toxicity test. Important behavioral changes dependent of the concentrations of MPA were also observed. There were no changes in the 2.5 and 5 mg/L treated groups, however, erratic swimming was observed in the groups exposed to median concentrations of MPA, and a significant reduction in the motor activity was observed at the higher concentrations. With respect to the effects on the gestation, none of the fish in the MPA-exposed groups, gave birth to any offspring. In contrast, all the fish in the control group grave birth to live offspring. In this group, two fish deliver its offspring after 48 hours (24 offspring) and in the other 4 fishes of the control group, the delivery occurred 3 and 5 days after exposure (41 offspring). A total of 65 offspring were born in the control group. Reduction in abdomen size and disappearance of the dark gravid spot occurred between 7 to 10 days in all fishes treated with MPA. Our Data indicates high chronic toxicity of MPA on fish reproduction, since gestation was totally interrupted at all concentrations tested. These results suggest death and resorption of the products. The mechanismo of the reproductive toxicity cannot be deduced in this study. Nevertheless, if the changes on the motor behavior are also considered combination there is likely an association with oxidative stress at some level in the homeostasis of pregnant guppy.For ethical reasons, the evaluation of the effects of any substance during pregnancy in humans is not permitted. Thus, the models that simulate some characteristics of this stage, are relevant in the investigation of reproductive effects, in special, those that alter the endocrine system in some way. Due to their high susceptibility to exogenous stressors, fish are considered to be one of the main biomarkers of the health of aquatic ecosystems. The guppies (Poecilia reticulata), are an oviviparus species characterized by the procreation of live offspring. For this reason, this species may be an important alternative in the evaluation of effects of xenobiotic compounds, particularly those considered to be caused by exposure during pregnancy. "Endocrine Disruptors" are substances that have the ability to alter the hormonal system, and consequently, could be hazardous to the reproductive health of various fish species. Multiple lines of evidence call attention to the high contamination of the aquatic environment with substances of pharmaceutical origin and their deleterious effects on the Endocrine System. Drugs that contain hormonal components should be given particular attention. Synthetic progestins are a large family of hormones derived from progesterone, which are widely used in human and animal contraception, and in hormone replacement therapies. Some studies have documented the endocrine disrupting effects of these compounds, however, there is still scarce knowledge about the effects of progestins the aquatic biota. The effects of Medroxyprogesterone Acetate (MPA) on gestation in guppy fish were evaluated. An acute toxicity test was conducted in seven groups of six pregnant female guppy fish, which were exposed to MPA concentrations of 2.5 mg/L, 10 mg/L, 100 mg/L, 150 mg/L and an untreated group (control group). Dilutions in 0.9% normal saline solution were prepared from 150 mg/mL injectable formulations of MPA. The tests were performed following the OECD guidelines for toxicity tests in a fish. Adittionally, all experimental activities were carried out in strict adherence to the biosafety standard established by the MINSA Technical Team of Panama. No individual in the control water group, nor in the different concentrations of MPA tested groups, died during the 96 houres after exposure in the acute toxicity test. Important behavioral changes dependent of the concentrations of MPA were also observed. There were no changes in the 2.5 and 5 mg/L treated groups, however, erratic swimming was observed in the groups exposed to median concentrations of MPA, and a significant reduction in the motor activity was observed at the higher concentrations. With respect to the effects on the gestation, none of the fish in the MPA-exposed groups, gave birth to any offspring. In contrast, all the fish in the control group grave birth to live offspring. In this group, two fish deliver its offspring after 48 hours (24 offspring) and in the other 4 fishes of the control group, the delivery occurred 3 and 5 days after exposure (41 offspring). A total of 65 offspring were born in the control group. Reduction in abdomen size and disappearance of the dark gravid spot occurred between 7 to 10 days in all fishes treated with MPA. Our Data indicates high chronic toxicity of MPA on fish reproduction, since gestation was totally interrupted at all concentrations tested. These results suggest death and resorption of the products. The mechanismo of the reproductive toxicity cannot be deduced in this study. Nevertheless, if the changes on the motor behavior are also considered combination there is likely an association with oxidative stress at some level in the homeostasis of pregnant guppy.