The carcinogenic effects of N-methyl-N-nitrosourea [MNU] after repeated i.p. administration in inbred strain NIH 13 guinea pigs were studied. Fifty percent of the animals that survived beyond 22 wk, after i.p. injection of MNU at a dose of 10 mg/kg body weight/week for 18 wk, developed a broad spectrum of tumors including adenocarcinoma of the pancreas in 2 animals, fibrosarcoma of the mesentery in 2 animals, angiosarcoma of the mesentery in 2 animals, mesothelioma of the peritoneum in 1 animal and tumors of the small intestine in 3 animals. These studies indicate the susceptibility of different types of tissue to the local effects of MNU after i.p. administration.