The effects of alpha-naphthoflavone, an aryl hydrocarbon receptor (AhR) antagonist, on the reproductive toxicity and teratogenicity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were investigated. Pregnant C57BL/6J mice were orally administered alpha-naphthoflavone either once on gestational day 12 (GD12; 50 microg/kg) or for 6 days (GD8-GD13; 5 mg/kg/day) followed by an oral challenge with TCDD (14 microg/kg) on GD12. Cesarean section was performed on GD18 for the evaluation of maternal and fetal toxicities. TCDD caused severe fetal malformations including cleft palate (43.7%) and renal pelvic and ureteric dilatations (100%). The administration of alpha-naphthoflavone either in a single treatment or 6-days remarkably reduced the incidence of cleft palate to 27.6% and 26.5%, respectively. In addition, the degree of renal pelvic and ureteric dilatations caused by TCDD were significantly attenuated by repeated treatment of alpha-naphthoflavone. These results suggest that AhR antagonists such as alpha-naphthoflavone could be promising candidates for reducing the incidence and severity of fetal malformations caused by TCDD exposure in utero.