ABSTRACTEmbryotoxicity and teratogenicity of 5‐fluorouracil (5‐FU) and modulation of its effect by N‐acetyl‐L‐cysteine (NAC) andor phorone were evaluated in mice. Pregnant ICR mice were intraperitoneally injected with 25 mglkg of 5‐FU on day 11 of gestation (vaginal plug = day 0). Pregnant mice were pretreated with NAC at dose levels of 80, 160, and 320 mg/kg injected intravenously 2 hours before dosing with 5‐FU. Pregnant mice were killed on day 17 of gestation. Fetuses were examined for external malformations, especially limb malformations. Pretreatment with 160 mg/kg and 320 mg/ kg of NAC decreased the incidence and severity of oligodactyly induced by 5‐FU. There was little difference in maternal body weight gain, fetal mortality, and fetal weight between the 5‐FU group and the 5‐FU plus NAC groups. Pretreatment with phorone, a glutathione depleting agent, at dose levels of 160 and 320 mg/kg injected intraperitoneally, 4 hours before dosing with 5‐FU, increased the incidence and severity of oligodactyly induced by 5‐FU. Cotreatment with NAC at dose levels of 160 and 320 mg/kg decreased the incidence and severity of oligodactyly induced by 5‐FU and 80 mg/kg of phorone. Cotreatment with NAC 160 mg/kg could not suppress the augmentative effect of phorone on 5‐FU teratogenicity under the severe condition, that is, the excess amount of phorone such as 320 mg/kg. These results indicate that the teratogenicity of 5‐FU is mitigated with NAC pretreatment, and also the level of endogenous glutathione is one of the factors which significantly a