The separate and combined effects of protein deprivation and benomyl ((methyl 1-butylcarbomoyl)2-benzimidazole carbamate) exposure were studied in the pregnant rat fed a diet containing 24% (control) or 8% (deficient) casein throughout gestation. Within each diet group, subgroups were gavaged at 31.2 mg/kg body weight with benomyl or corn-oil carrier only on d 7-16 or 7-21 of gestation. No effects on the skeleton were seen. Benomyl exposure in the last 2 wk in dams fed the 24% casein diet resulted in a high incidence of fetal brain anomalies. This effect did not occur in those with benomyl exposure during the period of organogenesis only and was reduced in groups fed the protein-deficient diet. Exposure to benomyl in the last 2 wk in the protein-deprived rat resulted in a decrease in the weight of the fetal heart in excess of that attributable to diet alone. Lungs were a smaller portion of body weight in fetuses of benomyl-treated dams in both diet groups. The teratogenic effect on the brain in animals exposed to benomyl in wk 2 and 3 of gestation suggests that screening for teratogenic effects during organogenesis only may be insufficient.