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. At necropsy of the offspring which began on postnatal day 133, Harderian gland weight reduction and hydronephrosis were seen at the 4.17- and 12.5-mg/kg dose levels, while no effects were found in body, liver, testes, seminal vesicle, kidney, or lung weights. Results of the present study and earlier studies demonstrate that rats are more sensitive than mice to the teratogenic effect of nitrofen (Gray et al., Science, 215 (1982) 293 and Gray et al., Toxicol. Appl. Pharmacol., 67 (1983) 1). In general, nitrofen affects the same organ systems in rats as it does in mice, but the rank order of sensitivity of these effects differs from those described earlier in the mouse by Gray et al. (Toxicol. Appl. Pharmacol., 67 (1983) 1).