Methyl(acetoxymethyl)nitrosamine (DMN-OAc) was synthesized and tested for toxicity and carcinogenicity in rats to test the hypothesis that alpha-hydroxylation is required for metabolic activation of dimethylnitrosamine (DMN) to a reactive, proximate carcinogen. The acute median lethal doses (LD50) of DMN-OAc and DMN injected ip into 5-week-old male Sprague-Dawley (Charles River (CD) rats were determined to be 0.19 and 0.59 mmole/kg body weight or 25 mg DMN-OAc/kg and 44 mg DMN/kg body weight, respectively. Single ip injections of one-half the LD50 DMN-OAc (13 mg/kg body weight) in 5-week-old rats of both sexes resulted in a high incidence of epithelial tumors of the intestinal tract. Mean survival times for rats with intestinal tumors were 353 days for males and 433 days for females. Tumors were rarely found at other sites. DMN at equivalent toxic (one-half the LD50, 22 mg/kg) and molar (= one-sixth LD50, 7.0 mg/kg) dose levels, yielded (as expected) tumors of kidneys, lungs, and occasionally other organs, but at a much lower incidence. The finding of the potent carcinogenicity of DMN-OAc supported the postulate that alpha-hydroxylation of DMN in vivo generates a proximate carcinogen.