The influence of ethanol consumption on cleft palate induction by methylmercury, cortisone, and retinyl acetate was investigated in Swiss white mice. Consumption of 20% ethanol throughout gestation significantly increased the incidence of cleft palate compared to water-fed mice, when methylmercury was given on four consecutive days (days 9-12, 5 mg/kg of body weight). Ethanol also increased the incidence of cleft palate in mice given retinyl acetate (3,400 or 5,100 IU) on day 12, compared to retinol acetate-treated mice given water, but did not affect cleft palate induction by cortisone (2.5 mg/d, days 8-11). Ethanol significantly reduced fetal weight in the presence or absence of the three teratogens, but the results do not support a hypothesis that growth retardation is directly responsible for the potentiating action of ethanol. It may be that ethanol acts to increase cleft palate induction by some teratogens by retarding fetal developmental processes.