Since the early 1990s, environmental estrogens have been recognized as an important environmental threat. Wastewater of 10 aerated lagoon treatment facilities in Iowa was evaluated for estrogenic activity using a short-term caged fathead minnow exposure and a plasma vitellogenin (Vtg) assay. Plasma Vtg results indicated that wastewater entering the three-lagoon systems was estrogenic to male fish, but with serial passage through the lagoons, the estrogenic activity decreased to a level that was not sufficient to induce vitellogenesis. Wastewater retention time in the lagoons may have been a key treatment factor.;Feral fathead minnows captured at aerated lagoon wastewater treatment facilities (WWTFs) exhibited plasma Vtg trends similar to those of the caged fish. Incidence of ovotestes in feral fish was low (1 of 65; 1.5%) and similar to that of fathead minnows captured at a reference site (national wildlife refuge). The results of both the caged and feral fish studies indicate that effluents from aerated lagoon WWTFs are low in estrogenic activity, but that raw wastewater was estrogenic to fish. Thus, the potential exists for release of estrogenic effluents from these systems if the treatment is not complete.;The plasma Vtg response of male fish exposed to estradiol for 10 days was dose-dependent and predictable (R2 = 0.988) through the range of estradiol exposure concentrations tested. The lowest observed effects concentration (LOEC) for induction of plasma Vtg was 50 ng/L. The dose-response curve from this study may be used in conjunction with exposure of male fish to surface water or wastewater to estimate the magnitude of the estrogenic potency of the water in terms of "estradiol equivalents.";Atrazine did not cause overt reproductive toxicity to adult fathead minnows in a short-term reproduction assay. However, decreasing trends in relative testis weight, testis maturity, and percent embryo fertilization suggest that further investigation is warranted. Nearly all endpoints concerning fish exposed to estradiol (positive control) were significantly different from atrazine-exposed fish and control fish. The results suggest that atrazine did not have strong estrogenic effects and did not cause endocrine system disruption in fathead minnows at environmentally relevant concentrations.