In separate subchronic toxicity studies, male and female Sprague-Dawley rats received three water disinfectants in the drinking water for 90 consecutive days. The treatment levels were 25 mg/L, 100 mg/L and 175 mm/L and 250 mg/L for chlorine, and 25 mg/L, 50 mg/L, 100 mg/L and 200 mg/L for both monochloramine and chlorine dioxide. Controls received carbonated (ph buffered) drinking water. Water consumption for all three compounds decreased in a dose-related fashion with increasing concentration of disinfectant, most likely due to unpalatability. None of the disinfectants caused premature deaths in any concentration evaluated. The highest dose of chlorine tested (250 mg/L) was considered to be a no observable adverse effect level (NOAEL) as no body weight, absolute or relative organ weight, hematological, clinical or histopathological changes were observed in either sex. The monochloramine produced both decreased body and organ weights in both sexes (heart, liver, lung and spleen in males; liver, spleen and thymus in females) and a small decrease in red blood cell count and serum calcium in males, 200 mg/L. Thus, the concentration (circa 9-12 mg/kg-day) was considered the lowest observable effect level (LOAEL), while 100 mg/L was designated the NOAEL.