To investigate the possibility of a free radical mechanism for ethanol-induced teratogenesis, gestational day 8 mouse embryos were exposed for 6 hr in whole embryo culture to a teratogenic dosage of ethanol alone (500mg%) or in conjunction with an antioxidant, superoxide dismutase (SOD; 300 U/ml). For subsequent analysis, some embryos were examined at the end of this 6-hr period, while others were removed to control medium and cultured for an additional time period. Ethanol exposure resulted in increased superoxide anion generation and increased lipid peroxidation (as noted 6 hr after initial ethanol exposure) and in excessive cell death (as noted 12 hr after initial exposure) in the embryos. Following a total of 36 hr in culture, a high incidence of malformation, including failure of the anterior neural tube to close in 63% of the ethanol-exposed embryos, was noted. The ethanol-induced superoxide anion generation, lipid peroxidation, excessive cell death, and dysmorphogenesis were diminished in embryos co-treated with SOD, suggesting that the teratogenicity of ethanol is mediated, at least in part, by free radical damage. © 1995 Wiley-Liss, Inc.