Estrogens that enter into the water system, such as by sewage effluent, may affect the reproductive capabilities of aquatic animals. The guppy (Poecilia reticulata) was chosen as a model organism to test the level of disturbances in primary and secondary sexual characteristics. Male guppies were exposed to 17beta-estradiol (10 and 50 ng L(-1)), and 17alpha-ethinylestradiol (10, 50 and 200 ng L(-1)) for 3.5 months from birth to adulthood. Specimens were euthanized and the testes removed for histological examination, applied in conjunction with stereological technique. Alterations in the quantity of sperm, decreasing intensity of body coloration, and increased body weight were generally significant only in the group exposed to 200 ng L(-1) of 17alpha-ethinylestradiol. No significant changes were seen in the gonadosomal index (GSI). Quantification of the developmental stages of spermatogenesis indicated a trend of greater representation of later stages in specimens exposed to low doses of estrogens, and greater prevalence of early stages in those exposed to high doses of estrogens. Based on the results of this study, environmental estrogens do not seem to pose a reproductive threat to guppies. It is concluded that stereological histology can be considered a valuable tool in quantifying estrogenic effects upon fish gonads and that the guppy is perhaps not an ideal test organism for estrogenic exposure due to limited sensitivity.