The effects of the hepatocarcinogenic peroxisome proliferating hypolipidemic agents clofibrate (CF) and nafenopin (NF) on rat liver carcinogenesis initiated by N-2-fluorenylacetamide (FAA) were studied and compared with that of the neoplasm promoter phenobarbital (PB). Male F344 rats were fed 0.02% FAA for 8 weeks to induce hepatocellular altered foci, and were then given no chemical or equimolar amounts (0.03 mmol/kg diet) of the chemicals for 24 weeks in the diet. In groups of animals killed sequentially, 0.07% PB had a marked enhancing effect on FAA-induced foci, while 0.073% CF produced only slight enhancement and 0.093% NF produced none. At the end of the experiment, only PB increased the incidence and multiplicity of liver neoplasms. NF suppressed histochemical gamma-glutamyltranspeptidase activity in the abnormal hepatocytes of foci as well as in periportal hepatocytes. In homogenates of livers from rats fed NF, gamma-glutamyl-transpeptidase activity was reduced, and this occurred to a lesser degree with CF, whereas PB enhanced activity. NF also induced alkaline phosphatase activity in hepatocytes throughout the lobule, but not in altered hepatocytes, thereby making foci demonstrable in sections reacted for alkaline phosphatase. These findings thus reveal significant cell membrane effects of NF and CF and suggest that their involvement in hepatocarcinogenesis is more complex than a promoting action.