The teratogenicity of phenytoin may be mediated through a reactive electrophilic and/or free radical intermediate which, if not detoxified, may interact with fetal cellular macromolecules and initiate teratologic effects. Glutathione (GSH) maintains cellular physiological processes and detoxifies xenobiotic reactive intermediates. The role of GSH in protection against phenytoin embryopathy was studied by altering GSH homeostasis using 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of glutathione reductase. BCNU was administered to pregnant CD-1 mice on gestational day 12, in doses ranging from 10 to 50 mg/kg i.p., with or without phenytoin, 55 or 65 mg/kg i.p., given 4 and 24 hr after BCNU. BCNU alone in doses of 10, 25 or 50 mg/kg resulted in a dose-related increase in the incidence of resorptions, cleft palates and postpartum death (P less than .05), and in lowered fetal weight (P less than .05). Fetuses exposed to 50 mg/kg of BCNU had an array of gross abnormalities, and this dose was not used in subsequent studies. BCNU, 25 mg/kg, inhibited GSH reductase activity by 23% in the placenta (P less than .05) and by 30% in the embryo (P less than .05) at 4 hr after treatment. Embryonic, but not placental GSH reductase activity remained significantly inhibited at 24 hr after BCNU. A BCNU dose-related increase in the incidence of resorptions (P less than .0001) and postpartum death (P less than .05) was seen in groups treated with both BCNU and 65 mg/kg of phenytoin, compared to controls treated with either chemical alone.(ABSTRACT TRUNCATED AT 250 WORDS)