The reproductive toxicity of (+/-)-9-fluoro-2, 3-dihydro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-7H-pyrido[1,2,3-de] [1,4]benzoxazine-6-carboxylic acid (ofloxacin, Tarivid), a new antibacterial agent, was investigated in rats and rabbits after oral administration. Neither the male or female fertility nor the reproductive performance of the rats was affected by doses of up to 360 mg/kg. Ofloxacin elicited no evidence of teratogenicity when administered orally during the period of organogenesis to pregnant rats at doses of up to 810 mg/kg, or to pregnant rabbits at doses of up to 160 mg/kg. However, the female rats receiving 810 mg/kg showed salivation, dirty hair coats, soft stools, and decreases of body weight and food intake. The fetuses in the higher dose groups exhibited decreased body weight and retardation of ossification, and those in the highest dose group showed increased mortality and skeletal variations (cervical ribs, shortened 13th ribs). Further investigation of the fetal skeleton revealed that the critical period of the occurrence of skeletal variations was on days 9 and 10 of gestation. The occurrence of cervical ribs and shortened 13th ribs was not an indicator of teratogenicity when ofloxacin was administered at doses of up to 1600 mg/kg during the critical period. Moreover, the shortening of the 13th ribs was the only type of retardation of ossification degree. Decreases of maternal body weight and food intake, and increased mortality of fetuses were observed in rabbits at a dose of 160 mg/kg. In a perinatal and postnatal toxicity study in rats using doses of up to 360 mg/kg, no adverse effects were observed.