Male reproductive abnormalities may be due to an increased level of maternal estrogens affecting the developing fetus. Man is exposed to environmental estrogens in multiple ways: diet, drinking water, air, and skin, para-Nonylphenol (p-NP), an alkylphenol, has estrogenic properties. This study was performed to evaluate the potency and workable concentrations before reproductive tests could be performed. The oral LD50 (median lethal dose) value for rats was 1475 mg/kg. For both males in the reproductive test the litter size after long-term treatment at 50 mg/kg p-NP was smaller than that in the breeding records. Five females did not conceive at all.