Reproductive and developmental toxicity studies of orally administered telithromycin (TEL) were carried out in SD rats and Himalayan rabbits. At first, in studies on fertility and embryonic development to implantation in rats, TEL of 50, 150 and 300 mg/kg/day were administered orally throughout pre-mating period, mating period in males, and in females, throughout pre-mating period, mating period and during pregnancy, until implantation. Although mating was not affected at any dose-level, fertility index, testicular head sperm count and daily sperm production rate in males, and fertility index in females were reduced at 150 and 300 mg/kg/day, and the number of corpora lutea was also reduced at 300 mg/kg/day. Consequently, under our experimental conditions, it is estimated that the No Observed Adverse Effect Levels (NOAEL) on paternal and maternal tolerance are 150 mg/kg/day and NOAEL on the development and maturation of gametes is 50 mg/kg/day. It is also considered that NOAEL on mating behavior and fertilization are 300 and 50 mg/kg/day, respectively and NOAEL on pre-implantation and postimplantation loss is 300 mg/kg/day. In the studies on rat embryo-fetal development, TEL was administered orally at dose levels of 50, 150 and 300 mg/kg/day during the period of fetal organogenesis in rats. Pregnant females showed slight signs of toxicity at 150 mg/kg/day, and increased at 300 mg/kg/day. No significant disruption of development and dysmorphogeny of offspring were observed at 50 and 150 mg/kg/day. Anomalies (incomplete ossification of cranium cervical vertebrae and extremities of paws) and malformations (bent ribs) were observed in fetuses born from the group treated with 300 mg/kg/day. These effects were associated with maternal toxicity. It is considered that in terms of maternal tolerance NOAEL is 150 mg/kg/day and in terms of embryo-fetal development NOAEL is also 150 mg/kg/day. In the studies on rabbit embryo-fetal development, rabbits received daily doses of 20, 60 and 180 mg/kg/day of TEL from the 6th to the 18th day of pregnancy. Dose-dependent reduction of food intake and body weight gain was detected in the dams from doses of 60 mg/kg/day. The hematological and blood biochemical parameters were not impaired by TEL except increased ALP at 180 mg/kg/day. In addition, autopsy of the dams did not reveal any macroscopic changes in the liver and kidneys. Apart from marginally delayed growth of fetuses at 180 mg/kg/day, the embryo-fetal development was not disturbed, and the intrauterine death rate was not increased. In morphological examination of the fetuses, no effects were observed on the findings of variation, anomaly and delayed ossification, and indications of teratogenic effect. The plasma levels of TEL in mated females were apparently dose-related. Plasma concentrations declined rapidly after treatment and there was no evidence of accumulation after repeated dosing. Fetuses, too, were exposed to TEL at a low level. Based on the results of this study, it is evaluated that NOAEL on mated female andembryo-fetal rabbits are 20 and 60 mg/kg/day, respectively. In the studies on pregnant, lactating female and on the development of the conceptus and offspring, the test compound was administered to pregnant rats from implantation through lactation at dose levels of 50, 125 and 200 mg/kg/day.