The i.m. administration of cortisone acetate, 2.5 mg., to nulliparous CF-1 mice, on Days 11–14 or 10–13 of gestation, yielded 75 and 70 percent incidences, respectively, of cleft palate in the 18-day fetus. The single i.p. injection of either cobaltous chloride, 25 mg./kg., or sodium cobaltinitrite, 50 mg./kg., on Day 10 or 11, followed by four daily i.m. administrations of cortisone or physiological saline caused a marked reduction in the incidence of fetal clefts in the presence of the steroid, and a significant increase in this anomaly in the presence of saline; the more profound effect in each instance being associated with the earlier day of administration. Replacement of cobaltous chloride with nickel chloride in the treatment regimen neither induced cleft palate with physiological saline nor markedly inhibited clefts caused by cortisone.