The subchronic toxicity of 0.2-200 ppm dichloramine and 0.2-90 ppm trichloramine in the drinking water of rats was investigated using biochemical, hematological, and histopathological parameters. Animals in the highest dose groups consumed 5-15% less fluid than controls with no significant decrease in body weight gain. No clinical signs of toxicity were observed in either case. Both males and females dosed with 90 ppm trichloramine had significantly increased relative kidney/body weights and the females had increased hepatic glutathione S-transferase and UPD-glucuronosyltransferase activities. No significant changes were detected in other xenobiotic metabolizing enzymes or in serum biochemistry, urine biochemistry, or hematology. Both dichloramine and trichloramine induced minimal to mild adaptive histopathological changes in thyroids and kidneys of animals of both sexes. Dichloramine, but not trichloramine, was associated with histological changes in the gastric cardia characterized by epithelial hyperplasia at concentrations of 2 ppm and above in the males and 200 ppm in the females. This study indicates that dichloramine produced mild histological effects at drinking water concentrations of >0.2 ppm in males (0.019 mg/kg/day) and >2 ppm in females (0.26 mg/kg/day) while trichloramine produced biochemical and mild histological effects at levels of >2 ppm both in males (0.23 mg/kg/day) and in females (0.29 mg/kg/day).