Two crude oils, differing in viscosity (V) and nitrogen (N) and sulfur (S) content, were evaluated for pre- and postnatal developmental toxicity. In Crude I (low V, low N, low S) studies, the material was applied neat to the clipped backs of pregnant rats at dose levels ofO, 125, 500, WOO (postnatal only), and 2000 (prenatal only) mg/kg. In Crude II (high V, high N, moderate S) studies, the oil was applied similarly but at dose levels of 0, 30, 125, and 500 mg/kg. Rats were exposed to the crude oils on gestation days (CD) 0-19; application sites were not covered. “Prenatal” rats were killed on CD 20. “Postnatal” rats were allowed to deliver naturally; surviving dams and litters were killed 3-4 wk postpar- turn. Both crude oils produced maternal and developmental toxicity. Adverse fetal effects included increased in utero death, decreased body weight, and reduced ossification of skeletal elements. Parturition was delayed in Crude II dams at 500 mg/kg. The 4-d viability index was decreased in all Crude ll-exposed groups. Pup body weights were decreased by each oil, but at the high dose only. Prenatal effects are probably related to polynuclear aromatic compounds (PAC) found in petroleum. The cause(s) of delayed parturition and postnatal toxicity have not been determined.