N-Nitrosocarbaryl was administered orally to 31 male Sprague-Dawley rats at doses of 130 mg/kg body weight (b.w.) twice weekly. Of the treated animals 29% died with squamous cell carcinomas of the forestomach after an average inductiom time of 167 days. The first carcinoma was observed as early as 63 days after the beginning of the experiment. In addition, 19% of the treated animals died with hyperkeratoses and 6% with papillomas in the forestomach. In contrast to the untreated controls, which had a normal life expectancy, treated animals had a very short life span due to high cumulative toxicity and the early appearance of tumors.