Fifteen male Beagle dogs received N-methyl-N-nitro-N-nitrosoguanidine (MNNG), 50 or 83 microgram/ml, in drinking water for 35 to 63 weeks. Broad superficial erosion and ulceration were observed in the angulus of the antrum and the anterior or posterior wall of the fundus during MNNG administration. After the discontinuation of MNNG administration, the erosions and ulcers healed rapidly, resulting in mucosal atrophy and scarring of the ulcer. In two dogs new depressions with atypical glands were observed by endoscopy and biopsy of the ulcer scars of the angulus, which became carcinomatous lesions at about the 100th week. Necropsy revealed 5 other carcinomas in the fundus of 5 additional dogs. One lesion was located in the ulcer scar and the other 4 in the areas of the mucosal atrophy. The possible relationship between carcinoma and the associated lesions was discussed.