The chlorination of surface waters is known to form trihalomethanes. Therefore, chlorine dioxide (CIO2) is being considered as an alternative disinfectant. This study was designed to determine the effect of chlorine dioxide and its metabolites, chlorite (CIO2) and chlorate (CIO3), on rat fetuses exposed in utero. Female rats were administed CIO2 at 0,1,10 and 100 mg1-1 and or CIO3 at 1 and 10 mg 1-1 daily in the drinking water for 2 1/2 months prior to and throughout gestation. Rats were killed on day 20 and fetuses examined for external, skeletal and visceral malformations. Slight decreases in weight gain during pregnancy were seen in the CIO2 administered groups. A significant dose-response relationship in the decreases of the numbers off implants and live fetuses were observed in the CIO2 groups.