Fibroblast Growth Factor (FGF) stimulates quiescent Swiss 3T3 cells to initiate DNA synthesis and divide. Cells begin to enter the S-phase after a lag of 13--15 hr, and the rate of initiation of DNA synthesis in the population can be quantified by a first order rate constant, k. A subsaturating concentration of FGF may establish the lag phase, while the value of k is dependent on the FGF concentration present during the second half of the lag phase. Insulin and hydrocortisone enhance the effect of FGF by increasing k without changing the lag phase, and they can act when added at any time after FGF. Prostaglandin E1 (PGE1) causes a decrease in k and a lengthening of the lag phase, and acts only when added during the first 8 hr. None of these agents stimulate DNA synthesis in the absence of FGF.