Human hepatocytes stimulated with human recombinant hepatocyte growth factor (h-rHGF) (10 ng/mL) displayed a characteristic lag period before entering into the S phase. The duration of this delay was dependent on the timing of h-rHGF addition to cultures. The highest peak of DNA synthesis was observed at 120 hours of culture when hepatocytes were stimulated with h-rHGF at 72 hours of culture. This was accompanied by an early peak of c-jun and c-fos synthesis (3 hours after addition of h- rHGF) followed by c-myc (6 hours) and increased expression of cyclins A, B, D, and E (12 hours after h-rHGF). A significant dose-dependent increase in inositol 1,4,5-P3 was observed within 45 seconds after stimulation with the factor. This was followed by an immediate increase in the cytosolic-free calcium. Cyclic adenosine monophosphate (cAMP) levels did not change after stimulation with the factor. Tyrosine phosphorylation seems to be an early event in the course of the stimulatory effect of h-rHGF on DNA synthesis of hepatocytes; genistein, a tyrosine kinase inhibitor, impaired the stimulatory effect of h-rHGF on DNA synthesis dose dependently. On the other hand, the action of the factor was negatively regulated by protein kinase C activation, as shown by the increased stimulatory effect of h-rHGF on DNA synthesis upon inhibition of protein kinase C by H7.