The comparative carcinogenicity of dimethylcarbamoyl chloride (DMCC) was studies in male, Syrian Golden Hamsters by inhalation. Hamsters were exposed to 1ppm and the exposure periods were 6 hours per day, 5 days per week for the lifetime of the animals. Fifty-one percent of the hamsters developed carcinomas of the nasal tract. Morphologically, all of these tumors were classified as squamous cell carcinomas. In comparing the response of rats and hamsters at 1 ppm DMCC, the rat seems to show increased sensitivity and the percentage of tumor yield is almost doubled, with the tumors appearing much earlier in rats than in hamsters. However, the remarkable tumor yield in both species indicates the potent carcinogenic effects of DMCC.