2,2'-Methylenebis (4-methyl-6-tert-butylphenol), an antioxidant, was given orally to pregnant Wistar rats by stomach intubation at the dose levels of 93.5, 187 or 375 mg/kg body weight during days 7 to 17 of pregnancy, and the effects of the compound on dams and fetal developments were examined. In the dams at the two higher doses of 187 and 375 mg/kg, toxic signs such as hair fluffing and diarrhoea were observed, and their body weight gain and food consumption were suppressed. Two dams, which showed marked diarrhoea in the highest dose group, died. However, there was no evidence of fetal malformation attributable to treatment with the compound in any of the dose groups treated, although a slight increase in fetal death was found in the highest dose group. It is concluded that 2,2'-methylenebis (4-methyl-6-tert-butylphenol) has a weak lethal effect on fetal development but not a teratogenic effect in the rat.