Intestinal tumors were induced by treatment with dimethylhydrazine (DMH) (14 mg with reference to base/kg body weight, weekly, s.c., for 20 weeks) in male rats supplied by Rappolovo Animal Farm. Some of the animals received 25 mg/day of misclerone (clofibrate) per os, 5 times a week. Clofibrate treatment did not affect the frequency of tumors of the large and small intestine but was followed by a significant decrease in the number of large- and medium-size tumors. The animals which had received clofibrate beginning from 10 days before the first injection of DMH revealed a relatively greater fraction of exophytic intestinal tumors and less invasion of the intestinal wall as compared with the animals treated with DMH alone. Possible mechanisms of the effect of clofibrate are discussed.