Chlorodibromomethane, a trihalomethane found in water supplies after chlorination, was administered by gavage in corn oil to male and female Fischer 344/N rats and B6C3F1 mice in toxicity studies of 13 weeks and 2 years duration. Doses used in the 13-week study were 0, 15, 30, 60, 125, and 250 mg/kg in rats and mice. At 250 mg/kg, hepatic and renal toxicity was produced in male and female rats and male mice, and mortality was increased in male and female rats. In the chronic study, male and female rats were administered the chemical at 0, 40, and 80 mg/kg. No adverse effects on survival in treated rats were observed. Male rats receiving 80 mg/kg had reduced body weight gains relative to controls. Fatty change and cytoplasmic changes were seen in the liver of treated male and female rats. The male and female mice in the chronic study received doses of 0, 50, and 100 mg/kg of chemical. A dosing accident rendered the number of low-dose male mice inadequate for statistical analysis. High-dose male mice had reduced survival relative to controls. Survival was similar in dosed and control female mice. Dosed male and high-dose female mice had reduced body weight relative to controls. Non-neoplastic hepatic lesions were seen in treated male mice (necrosis and hepatocytomegaly) and in treated female mice (calcification and fatty change); nephrosis was seen in treated male mice. The incidence of hepatocellular adenoma and carcinoma (combined) was increased in treated female mice, but only marginally so in treated male mice. Chlorodibromomethane, like chloroform, is toxic to liver and kidneys, and like chloroform induces hepatocellular tumors in mice.