ABSTRACT— Patients with hepatic tumours have increased serum activities of alkaline phosphatases. In order to clarify the origin of the increased enzyme activity, two experimental models of rat liver carcinogenesis were studied. In one model, the resistant hepatocyte model, the process was initiated by diethylnitrosamine (DEN, 200 mg/kg), and preneoplastic liver nodules were selected for by 2 weeks of 2-acetylaminofluorene (2-AAF, 0.02%) feeding and partial hepatectomy (PH). High activities of serum alkaline phosphatases were found in these rapidly growing nodules harvested at the peak time of nodular mass expansion. In the other model in which nodules were induced with long-term intermittent feeding of a diet containing 0.05% 2-AAF and harvested in a late stage with a low increase of nodular liver cell mass, no such increase in serum alkaline phosphatase activity was found. A similar difference was also noted when measuring the activities of the enzyme in the nodular tissue. Thus the first model showed high activities of the enzyme in the nodular tissue, while the second model had similar activities to those of the control animals. The serum levels reflected the nodular enzyme activity in both models. The tissue surrounding the nodules did not show increased enzyme activity. No difference was noted in the serum or tissue activity of transaminases. In both models the liver nodules occupied 30–50% of the liver volume. The experimental models were selected to emphasize the importance of the rate of intrahepatic mass expansion for the levels of serum alkaline phosphatase activities. Our findings indicate that the increase in serum activities of alkaline phosphatase was caused by induction and release of the enzyme in the nodules themselves rather than by tissue compression and a following cholestasis of the surrounding liver tissue.