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, ClO2(-1) and ClO3(-1) (10, 100 mg/L) in drinking water in rats. After 9 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 months no significant hematologic changes were noted in treated rats compared to control. However, after 9 months RBC counts, hematocrit, and hemoglobin were decreased in all treatment groups. ClO2, ClO2(-1), and ClO3(-1) administered chronically in drinking water for 3 months inhibited the incorporation of (3)H-thymidine into nuclei of rat tests. The treatment with Cl compounds decreased rat body weight in all groups after 10 and 11 months treatment.