Nitrosomethyl-n-dodecylamine, a product of the reaction of dimethyl-n-dodecylamine with nitrous acid, was given to Sprague-Dawley rats by gavage in olive oil solution. Twice-weekly treatments with 12 mg of the nitrosamine for 50 weeks gave rise to 100% incidence of transitional cell carcinomas of the urinary bladder. In contrast with the tests of other N-nitroso compounds, this compound seemed to be more effective in males than in females; females died later with the tumor although the dose per unit body weight was higher in females than in males.