Mitoinhibitory effects of clofibric acid (CA) were investigated in rat hepatocytes in vitro and in vivo. Female Sprague-Dawley rats, 11 weeks of age, were dietetically treated with CA at 9000 ppm for up to 13 weeks. In the in vitro study, hepatocytes were isolated from rats on day 4 and in weeks 1, 2, 5, 9 and 13, and cultured for 48 h in a medium containing epidermal growth factor. Over the last 24 h, bromodeoxyuridine (BrdU) was added to the medium to determine number of hepatocytes in S-phase. The labeling index (LI) of CA-treated hepatocytes in culture was below control values as early as day 4, and progressively declined with increasing treatment period. In the in vivo study, rats were 2/3 partially hepatectomized at week 13 and maintained for 24 h. BrdU was injected intraperitoneally 1 h prior to necropsy. The LI in the CA-treated liver was also decreased. These data indicated that continuous treatment of CA resulted in decreases in hepatocyte response to growth stimuli, and suggested a possible relation between the chronic growth inhibitory effect on cell multiplication and tumor promotion in rat hepatocarcinogenesis.