Ethyl carbamate (EC) is a potent teratogen in rodents and is present at low concentration in fermented foods and alcohol beverages. It has been well hypothesized that some metabolic products are responsible for the teratogenic effects of the compound. In the present study, the effects of phenobarbital (PB) on EC-induced embryotoxicity were investigated in SD rats. Six groups were constructed: EC 300 (EC 300 mg/kg/day), EC 600 (EC 600 mg/kg/day), EC 600+PB (EC 600 mg/kg/day and PB 80 mg/kg/day), PB (PB 80 mg/kg/day), DR (dietary restriction, 8 g/day/rat) and a control group. Rats of the EC 600+PB group were pretreated with phenobarbital intraperitoneally for three days to induce cytochrome P450 enzymes, followed by oral administration of EC for two consecutive days. The incidence of fetal deaths in the EC 600+PB group was higher than that of the EC 600 group(42.7 vs. 14.3%). The incidence of fetal realformations in the EC 600+PB group was higher than that of the EC 600 group (external; 7.0 vs. 4.1%, visceral; 31.4 vs. 11.3%, skeletal; 11.1 vs. 6.5%). There was no embryotoxicity in the control, EC 300, PB and DR groups. These results show that the pretreatment with phenobarbital augments EC-induced embryotoxicity in rats, indicating an evidence that metabolic activation by cytochrome P450 may be the major pathway of EC to its embryotoxic forms.