||Our earlier studies have revealed that direct hyperplasia induced by liver mitogens such as lead nitrate, ethylene dibromide, nafenopin and cyproterone acetate, unlike compensatory cell proliferation induced by partial hepatectomy and CCl4, does not support the formation of enzyme-altered islands induced by chemical carcinogens in the liver. In the previous studies carcinogens were given at the peak of DNA synthesis induced by the liver mitogens. If the mitogens have altered the sensitive phase of the hepatocyte to the carcinogenic attack, administering the carcinogen at one time point following the mitogenic stimulus might have missed the sensitive phase. In order to overcome this possibility in the present study male Wistar rats weighing 200-250 g were given N-methyl-N-nitrosourea (MNU; 60 mg/kg, i.p.) at three points representing G1, S and G2/M phases of the cell cycle following different types of liver cell proliferative stimuli. In another experiment MNU (60 mg/kg, i.p.) and diethylnitrosamine (15 mg/kg, i.p.) were given prior to the administration of proliferative stimuli. The initiated hepatocytes were also assayed following promotion by two different promoting regimens, namely phenobarbital and the resistant-hepatocyte model. Further, the initiated hepatocytes were monitored not only by using the appearance of islands of enzyme-altered hepatocytes but also using the incidence of hepatocellular carcinoma. The results of this study clearly revealed that irrespective of the protocol used, only the compensatory liver cell proliferation but not the mitogen-induced direct hyperplasia supported the formation and the growth of enzyme-altered islands in the liver induced by chemical carcinogens.