Carcinogenicity and chronic toxicity of ortho-chloronitrobenzene (o-CNB) were examined by feeding groups of 50 F344 rats and 50 BDF(1) mice of both sexes o-CNB-containing diets for 2 years. The dietary concentration of o-CNB was 0, 80, 400 or 2000 ppm (w/w) for rats and 0, 100, 500 or 2500 ppm for mice. The 2-year administration of o-CNB produced a dose-dependent increase in incidences of hepatocellular adenomas and carcinomas in rats and mice of both sexes and hepatoblastomas in mice of both sexes. Incidences of altered cell foci in the liver were increased in the o-CNB-fed rats of both sexes. Metastasis from mouse malignant liver tumors occurred predominantly in the lung. The hepatocarcinogenic response to o-CNB was found to be more potent in mice than in rats. Marginally increased incidences of renal cell adenomas in the 2000 ppm-fed female rats and renal cell carcinomas in the 2000 ppm-fed male rats were noted, together with a significantly increased incidence of atypical tubule hyperplasias. Spontaneous, age-related chronic progressive nephropathy was exacerbated in a dose-related manner, and caused the death of 47 male rats fed 2000 ppm before the end of the 2-year administration period. The highest dose levels of o-CNB except for the administration of 2000 ppm to male rats were thought to meet the criteria of the maximum tolerated dose set by both NCI and IARC guidelines. Causative factors of o-CNB-induced carcinogenicity were discussed with reference to our previous rodent studies of subchronic toxicity of o-CNB and carcinogenicity and chronic toxicity of para-chloronitrobenzene.