A fertility study was performed in rats by intraperitoneal administration of cefpirome sulfate (CPR) at dose levels of 0 (contro1), 200, 400 and 800 mg/kg/day. Male rats were treated for 60 days before mating and during the mating period. Female rats were administrated the substance from 14 days before mating to day 7 of pregnancy. The females were sacrificed on day 21 of pregnancy for examination of their fetuses. No animal died during the administration period. Loose feces were observed in both male and female animals in the 400 and 800 mg/kg/day groups. Body weight gain was suppressed in both male and female animals in the 800 mg/kg/day group and in male animals in the 400 mg/kg/day group. There were no significant differences in food and water intakes between treated and control groups. Autopsy revealed the enlargement of the caecum in males in the 200, 400 and 800 mg/kg/day groups. The kidney and adrenal weights were significantly increased in males in the 200, 400 and 800 mg/kg/day groups. Fertility and reproductive ability in both sexes, and estrus cycles in female rats were not affected by administration of CPR. There were no lethal effect and growth-inhibiting or teratogenic effects on the embryos and the fetuses. The results suggest that the non-effective dose level of CPR was lower than 200 and 200 mg/kg/day for general toxicity in male and female parent animals respectively, 800 mg/kg/day for reproductive ability in parent animals and in embryos and fetuses.