A perinatal and postnatal study of KTC-1, a new semisyntheitic rifamycin antituberculous drug, was conducted in Sprague-Dawley rats. Dosages of KTC-1 0, 12, 27.6, and 63.5 mg/kg/day were administered to dams orally by gavage from day 17 of gestation to day 21 of lactation. All pregnant rats were allowed to deliver naturally for postnatal examination of their offspring. At 63.5 mg/kg/day, weakness, dark-red discharge around eyes, a loss in body weight, and a decrease in food and water consumption were observed in dams. An increase in the weight of adrenal gland and spleen, and a decrease in the weight of kidney and heart were also found. An increase in neonatal deaths during the lactation period, a loss in body weight, a delay in physical development, a decrease in traction ability, an increase in the number of errors and the time required for the multiple T-maze trial were found in F1 offspring. In addition, an increase in the incidence of visceral variations and retarded ossification were observed in F1 4 day old rats. An increase in the incience of skeletal anomalies was seen in F2 fetuses. There were no sings of maternal toxicity or embryotoxicity at 12 and 27.6 mg/kg/day. From the results mentioned above, it can be concluded that the no-effect dose levels(NOELs)for dams, F1 offspring, and F2 fetuses are 27.6 mg/kg/day.