Primary monolayer cultures of adult rat hepatocytes were used to study the temporal interaction of epidermal growth factor (EGF) and insulin in their stimulation of DNA synthesis. The hepatocytes were cultured both under defined conditions and with serum. EGF and insulin interacted synergistically. The entry into S phase (G1 exit) followed first-order kinetics both in untreated and hormone-stimulated cells. Addition of EGF and insulin at the time of plating did not alter the lag period before the DNA synthesis started (25-26 h), but the rate constant for the S phase entry increased five- to sixfold. Experiments where the time of hormone addition was varied indicated that insulin exerted its strongest effect at the time of plating, whereas the cells became more responsive to EGF after being cultured for up to 40-50 h. The responsiveness to EGF at these later stages required an early exposure of the hepatocytes to insulin. When the administration of EGF to insulin-pretreated hepatocytes was postponed for 44 h after plating in serum-free medium, the cellular sensitivity was increased as compared to EGF treatment at 0 h (a one-log shift of the dose-effect curve), the rate of S phase entry was more rapid, and the lag period for the onset of the EGF effect (i.e., shift of rate constant) was shortened (6-7 h vs. 26 h).