Nature of insulin-like growth factor (IGF) binding on rat hepatocytes of primary monolayer culture and its regulation by IGF-related peptides were studied. The specific binding of 125I-IGF-I to the rat hepatocytes was saturable and dependent on incubation time and temperature. The optimal pH of incubation medium was 7.8-8.2. The steady-state binding was attained at 20 degrees C for 6 h, and averaged 12% per 4 X 10(5) cells. Fifty percent displacement of the 125I-IGF-I binding was observed at concentrations of 10 ng/ml of IGF-I and 5 ng/ml of IGF-II, but insulin (10 ng/ml-10 micrograms/ml) did not inhibit the binding. IGF-II inhibited the binding of 125I-IGF-II to the hepatocytes half maximally at concentrations of 2 ng/ml. A Scatchard plot of the data of binding was linear, and indicated association constant of 1.0 X 10(9)M-1 and a binding site of 44,000 per cell. Preincubation of the hepatocytes with either IGFs or insulin at 37 degrees C for 24 h resulted in the reduction of 125I-IGF-I binding to the hepatocytes in a dose-related manner, being dependent on incubation time and insulin concentrations. The binding was reduced to 20% of the control value in the presence of 10(-7)M IGF-I, and to 85% of the control in the presence of 5 X 10(-8)M IGF-II. Although insulin showed no affinity to the type II IGF receptors, 125I-IGF-I binding to the hepatocytes was decreased to 60% of the control after preincubation with 10(-10)M insulin. These results indicate that 1) the hepatocytes of primary monolayer cultures have only type II IGF binding, 2) preincubation of the cells with IGF-I, IGF-II or insulin induces the reduction of 125I-IGF-I binding to the cells, indicating down-regulation of the binding, and 3) this binding site of rat hepatocytes is regulated by not only IGF-I and IGF-II but also insulin of physiological concentrations.