Nitrofen was administered to pregnant Sprague-Dawley rats by gavage on days 8-16 of gestation at 5 different dose levels - 0, 0.46, 1.39, 4.17 and 12.5 mg/kg/day. Diaphragmatic hernias were found in pups that died immediately after birth at the 3 highest dose levels. At the 1.39 mg/kg dose level 3 of the 4 pups examined had diaphragmatic hernias, at the 4.17 mg/kg dose level 2 out of 3 pups had diaphragmatic hernias, and at the 12.5 mg/kg dose level all 5 pups found dead had diaphragmatic hernias. Locomotor activity of the offspring was measured on postnatal days 17 and 24, and hyperactivity was evident at the 3 highest dose levels. However, when the rats were later tested at 45, 49 and 90 days of age they had apparently recovered from this earlier hyperactivity. In the female rat, nitrofen did not delay the onset of puberty as measured by the age of vaginal opening or the age at first estrus.