The carcinogenic effectiveness of nitrosomethyl-n-butylamine,-n-hexylamine and -n-heptylamine was compared by administration to F344 rats at various concentrations in drinking water and by gavage in oil. Nitrosomethylbutylamine induced only tumors of the upper gastrointestinal tract, and was very toxic. Nitrosomethylhexylamine was less toxic and induced tumors of the liver and lung, as well as those the upper GI tract, when given by gavage, but only tumors of the GI tract when given in drinking water. In contrast, nitrosomethylheptylamine induced tumors of the liver and lung when given in drinking water or by gavage, but there were tumors of the esophagus only in those rats treated via the drinking water. As measured by the dose needed to produce a given carcinogenic effect, nitrosomethylheptylamine was a weaker carcinogen than the other two nitrosamines, but was not inactive as has been reported previously.