A comparison was made of the carcinogenic effectiveness of five hydroxylated nitrosodialkylamines given to female F344 rats in drinking water. Nitroso-2,3-dihydroxypropyl-2-hydroxypropylamine was the most potent carcinogen among those examined, and induced almost exclusively tumors of the upper gastrointestinal tract. Considerably less potent was nitrosobis-(2-hydroxypropyl)amine, which also induced mainly tumors of the esophagus, although several animals also had tumors of the nasal cavity. Nitrosodiethanolamine was weaker again, and induced a high incidence of hepatocellular carcinomas. Nitroso-2-hydroxypropylethanolamine, which can be considered a hybrid of nitrosodiethanolamine, and the bis-hydroxypropyl compound, induced both esophageal tumors and hepatocellular carcinomas, together with some liver angiosarcomas; it appeared to lie in potency between the two symmetrical compounds. In contrast, nitroso-2,3-dihydroxypropylethanolamine was a very weak carcinogen, having little life-shortening effect; it induced hepatocellular carcinomas in only a small number of animals, together with some neoplastic nodules.