Chlorine dioxide (ClO2) is currently being considered as an alternate to chlorine as a disinfectant for public water supplies. Studies were conducted to determine the toxicity of ClO2 (0, 1, 10, 100, 1000 mg/l) and its metabolites, ClO3- and ClO3- (10, 100 mg/l) in drinking water in rats. After nine months treatment the osmotic fragility of the red blood cells was decreased in all treatment groups, while a decreased blood glutathione was only observed in the metabolite groups. At 2, 4 and 6 mon no significant hematologic changes were noted in treated rats compared to control. However, after 9 mon RBC counts, hematocrit and hemoglobin were decreased in all treatment groups. ClO2, ClO2- and ClO3- administered chronically in drinking water for three months inhibited the incorporation of 3H-thymidine into nuclei of rat testes. Also, this inhibition was observed in the liver of ClO2- groups and in the kidney of 100 mg/l ClO2- treatment. The incorporation in small intestinal nuclei was increased in both 10 and 100 mg/l ClO2 and in 10 mg/l ClO2-. The treatment with Cl-compounds decreased rat body weight in all groups after 10 and 11 months treatment.