The effects of dietary 2-acetylaminofluorene (2-AAF) on cell cycle-related proteins was studied in regenerating livers from male Wistar rats, The levels of cyclins, cyclin dependent kinases (cdks), and related proteins were studied at different times during the first cell cycle after partial hepatectomy (PH). The frequency of proliferation cell nuclear antigen (PCNA)-positive nuclei, a marker of S phase progression, was almost zero during the first 27 hours after PH in the mitoinhibited 2-AAF-treated rats, while about 50% of the nuclei were labeled 24 hours after PH in control animals. Accordingly, Western blot tests showed markedly elevated PCNA protein levels from 18 hours to the end of S phase in untreated animals but no upregulation in response to 2-AAF. Compared with control animals, animals treated with 2-AAF showed increased levels of cdk 4 and cyclin D-3 from 12 and 15 hours after PH, respectively, and altered cyclicity in cyclin Dg expression. No effects on cyclin E were observed, while the increase in cdk 2 levels in control animals during late G(1)/S (15-27 hours) was abolished by 2-AAF. p53 was induced by 2-AAF treatment during the same period, with a peak at 24 hours. The protein detected with p21 antibodies was highly expressed in unstimulated hepatocytes in control animals, and further increased by 2-AAF. The expression was sustained until 15 hours after PH in control rats while 2-AAF-treated animals lacked detectable protein during this period; however, a transient increase was observed at 21 hours, Thus, 2-AAF affects several parameters of cell cycle regulation of possible relevance for its inhibitory effects on hepatocyte proliferation in vivo.