The fetal toxicity of potassium nitrate (KNO3) used widely as a food additive was studied in Wistar rats. The pregnant rats were fed a diet containing 2.5, 0.5 or 0.1% of KNO3 from day 7 to 14 of pregnancy. Neither maternal nor fetal toxicity including external malformations were observed at term in any group. After spontaneous delivery, the offspring were reared until 13 weeks after birth. No harmful effects were detected in any group. The female offspring (F1) of all groups were mated with the male (F1) of the same group. Good reproductive performances were shown in all groups. The pregnant rats were fed the same diet, which their mothers (F0) had been fed, from day 7 to 14 of pregnancy. Various types of malformations such as exencephaly, cleft lip and palate, polydactyly and micro- or anophthalmia were observed in 6 of 133 fetuses and 7 of 63 newborns from dams treated with 2.5% KNO3, but no external malformations were observed in other groups. The male offspring (F2) of the treated groups showed slow growth until 13 weeks after birth. These results suggest that a dose of 2.5% KNO3 is toxic to the F2 generation, but not to the F1 generation.