Pregnant A/J mice received 60 mg phenytoin/kg body weight on Day 10 of gestation. Eighteen hours after phenytoin injection, animals were injected (ip) with 20 microCi/g of [35S]methionine. After 6 hr of incorporation animals were sacrificed and the embryos were removed. Protein synthesis in the embryo, as measured by [35S]methionine incorporation into trichloroacetic-precipitable protein, was analyzed by SDS-PAGE and quantitation of autoradiograms. The results of gel electrophoresis indicate that in embryonic primary palates and limb buds from phenytoin-treated mothers there is an increase in synthesis of 66-, 50-, 44-, and 13-kDa proteins and a decrease in synthesis of an 18-kDa protein compared with values for the control counterpart. No role has been assigned to the 66-, 44-, or 13-kDa proteins but the 50-kDa band comigrates with tubulin and the 18-kDa band comigrates with calmodulin. Palatal cells in vitro stained positively with specific antibody to both these proteins. An adverse effect of the anticonvulsant drug phenytoin, when administered to pregnant A/J mice is an increase in the incidence of cleft lip with or without cleft palate [CL(P)] in their offspring. These alterations in protein synthesis may be a direct or secondary result of maternal phenytoin treatment and may play a role in CL(P) formation in vivo.