Enzyme-altered foci and nodules have been observed in the liver of rats treated with diethylnitrosamine(DENA) followed by selection with 0.02% 2-acetylaminofluorene (2-AAF) and a partial hepatectomy (PH) (DENA-2AAF-PH treatment). The authors determined the effect of subsequent administration of phenobarbital or of a second dose of DENA on the progression or reversion of these lesions. After the removal of 2-AAF from the diet, one group of rats received no further treatment, another group received subsequent exposure to phenobarbital, a third group received a second dose of DENA, and a fourth group received a second dose of DENA without the partial hepatectomy during the 2-AAF exposure. The rats were sacrificed at various times up to 58 weeks following the 2-AAF exposure. Subsequent exposure to phenobarbital did not alter the tumor incidence in the DENA-2AAF-PH treated rats while the second dose of DENA resulted in an increased incidence of hyperplastic nodules and hepatocellular carcinomas as well as an increase in renal tubular adenomas and carcinomas.