Nitrosocarbaryl, nitroso-N-methylurethane and nitroso-N-ethylurethane were administered by gavage in olive oil solution to groups of 12 female Sprague-Dawley rats. The dose was 0.2 ml of 0.11 M solution once a week for 10 weeks, a total dose of 0.22 mmol. The rats given nitrosocarbaryl survived longer, but had as high an incidence of tumors (75%) as did rats given nitrosomethylurethane. Most of the tumors induced were invasive squamous carcinomas of the stomach. Nitrosoethylurethane appeared to be a little more potent than nitrosomethylurethane; all 12 animals in this group had squamous stomach tumors at death. A higher total dose of nitrosocarbaryl, 1.3 mmol, given to male rats twice weekly for 20 weeks did not produce a higher incidence of stomach tumors than did thelower dose in females, although the males died earlier with tumors.