Leiomyosarcomas of the small intestine were found in dogs during experimental induction of gastric carcinoma by oral administration of N-methyl-N''-nitro-N-nitrosoguanidine (MNNG). Leiomyosarcomas were found most frequently in the duodenum and jejunum, and occasionally in the stomach but never in the ileum, colon or rectum. The leiomyosarcomas developed in all dogs given 50 .mu.g/ml of MNNG in deionized water to drink but not in dogs fed on porridge food made from standard pellet diet mashed with MNNG at the same concentration in tap water. Intestinal sarcomas developed in 3 mo.-5 yr after the end of MNNG administration, and frequently metastasized to the liver and/or the peritoneum.