Carcinogenicity of catechol, a naturally occurring and industrial chemical which has been shown to have strong cell proliferating potential on rat glandular stomach epithelium, was investigated in male and female F344 rats and B6C3F1 mice. Groups of 30 male and female F344 rats and B6C3F1 mice were treated with 0.8% catechol in powdered diet continuously for 104 weeks (rats) or 96 weeks (mice). At necropsy, neoplastic lesions were observed mainly in the glandular stomach of both species. Adenomas were found in all rats and in the majority of mice: 29 out of 30 (97%) in males and 21 out of 29 (72%) females. In addition 15 out of 28 (54%) and 12 out of 28 (43%) of the male and female rats respectively, had well differentiated adenocarcinomas. No adenocarcinomas were found in mice of either sex. In the forestomach epithelium, although significant increase in papilloma development was not evident, incidences of squamous cell hyperplasia were significantly increased in rats and mice of both sexes. In other organs examined, incidence and numbers of liver hyperplastic foci per cm2 liver section were significantly lower in male rats. Although the incidence was not different, the numbers of hyperplastic foci were also significantly reduced in female rats. Thus the present experiment clearly demonstrated that catechol exerts carcinogenic activity in rodent glandular stomach epithelium.