Single doses of noncarcinogenic (4-aminoazobenzene) and carcinogenic (4-dimethylaminoazobenzene and 3′-methyl-4-dimethylaminoazobenzene) azo dyes, as well as daily injections of thioacetamide, were administered intraperitoneally to male Wistar rats, which had been partially hepatectomized 3 or 7 days after the beginning of the experiments. In both control rats and rats treated with the carcinogens, 4-dimethyaminoazobenzene and thioacetamide, the nucleolar volumetric and numerical changes were closely related to the mitotic activity of the regenerating liver tissue, especially in the hepatic cells around the portal veins. Nuclear changes (chromosome bridges) and nucleolar alterations (anisonucleolinosis) were observed after the administration of liver carcinogens, their development being strongly related to and governed by a dose-response relationship. Cytochemical tests showed that nucleolar lysine-rich proteins increased simultaneously with anisonucleolinosis, especially after thioacetamide administration. Intranucleolar Feulgen-positive granules appeared after the administration of liver carcinogens. The possible significance of these morphological findings is discussed, and early damage of the hepatocyte nucleoli after the application of liver carcinogenic substances is suggested.