Nitrosoazetidine was fed to rats in drinking water at three different concentrations. At 2 mmol (10 mmol total dose) all of the rats died with hepatocellular carcinomas by the 62nd week. This was a greater response than to an equivalent dose of nitrosopyrrolidine. At 0.67 mmol, 6 of 21 rats developed liver tumors, and at 0.17 millimolar 2 of 30 animals had liver tumors. The comparable responses to feeding of nitrosoazetidine-2,2,4,4-d4 were 3 of 21 rats with liver tumors at 0.69 millimolar and 3 of 30 rats with liver tumors at 0.17 mmol. These results show only a small deuterium isotope effect in liver carcinogenesis with nitrosoazetidine.