An experiment was performed to test whether inclusion of phenobarbital in a choline-devoid diet would increase the hepatocarcinogenicity of the diet. Groups of 5-week old male Fischer-344 rats were fed for 7-25 months semipurified choline-devoid or choline-supplemented diets, containing or not 0.06% phenobarbital. No hepatic preneoplastic nodules or hepatocellular carcinomas developed in rats fed the plain choline-supplemented diet, while one preneoplastic nodule and one hepatocellular carcinoma developed in two rats fed the same diet containing phenobarbital. The incidence of preneoplastic nodules and of hepatocellular carcinomas was 10% and 37%, respectively, in rats fed the plain choline-devoid diet, and 17% and 30%, in rats fed the phenobarbital-containing choline-devoid diet. The results evinced no enhancement of the hepatocarcinogenicity of the choline-devoid diet by phenobarbital. Sporadic neoplastic lesions were observed in organs other than the liver of some of the animals, irrespective of the diet fed.