N-Nitrosopyrrolidine and 2 of its derivatives were prepared and fed in drinking water to Sprague-Dawley rats to compare the effects of substituents on the carcinogenicity of the N-nitrosopyrrolidine molecule. 3,4-Dichloro-N-nitrosopyrrolidine induced esophageal tumors in 13 of 14 animals, olfactory carcinomas in 4 and a heptocellular tumor in 1. All animals that received this compound were dead 55 wk after the start of the experiment. N-Nitrosopyrrolidine induced hepatocellular tumors in 26 of 29 animals and induced 1 olfactory carcinoma. Not all animals in this group were dead until 104 wk of the experiment. 2,5-Dimethyl-N-nitrosopyrrolidine induced only 2 hepatocellular tumors in 29 animals. .alpha.-Methyl substitution diminished liver carcinogenicity, while .beta.-Cl substitution affected a different target organ, the esophagus, and greatly reduced the the time to death with tumors.