Sodium colistin methanesulfonate (CLM) was intravenously injected to male and female Wistar strain rats, once daily, at doses of 6.25mg, 12.5mg and 25mg/kg to examine the influences on their fertility and development of fetuses. Male rats were treated with CLM for more than 60 days before mating until copulation. Female rats were treated with CLM for 14 days before mating, furthermore the treatment was continued until day 7 of gestation. Results were given as follows; 1. CLM did not affect general condition, body weight, food consumption, copulation and pregnancy. 2. CLM did not affect general condition, body weight and food consumption in preganant rats. 3. There was no significant difference between the CLM treated groups and the control group in number of implantations, number of live fetuses, weight of live fetuses, sex ratio, number of resorptions, number of dead fetuses and weight of placentas. 4. No internal and external malformation as well as anomalous ossifications of fetuses were observed in any treated group. 5. The ossifications of cervical vertebral centra, epiphysis of the caudal vertebral and proximal diaphysis of the hind paw were delayed by treatment with CLM at dose of 25mg/kg. It can be concluded that CLM has no unfavourable influence on fertility and fetal development in rats. In this study, the maximum non-effect dose of CLM was 12.5mg/kg in rats.