Eight months after a single s.c. injection of 1,2-dimethylhydrazine (21 mg/kg b.w.) to male Sprague-Dawley rats, seven colonic tumors (i.e. 17.5%) were found at autopsy. Histological examination revealed invasive adenocarcinomas in all seven tumors. All tumors were seen to originate in pre-existing discrete lymphoid aggregates. Adenomas or adenocarcinomas arising from pre-existing adenomas were not found. Previous experiments, using the same strain of rats, demonstrated the occurrence of adenomas with or without invasive adenocarcinomas after multiple doses of DMH. The causes responsible for the dose-dependent specificity in the histogenesis of colonic tumors in our model remains to be elucidated.