CHO-K1 cells grow in a defined medium with insulin, at physiological concentrations, as the only hormone. IGF-I can substitute for insulin. Quiescent cells require a 9-10-h lag, subsequent to the addition of insulin, to synthesize DNA. The phorbol ester, 12-O-tetradeconoylphorbol 13-acetate (TPA), cannot support growth of these cells, is a more effective inducer than insulin of c-fos, c-myc, c-jun, jun-B, Krox-20, Krox 24, fra-1 and JE, and induces fra-1, JE and c-myc with different kinetics from those of insulin. The addition of insulin + TPA to quiescent cells produces a synergistic effect on DNA synthesis but not on the expression of immediate early genes. Pretreatment of these cells with TPA or insulin decreases the required lag time for DNA synthesis by 3 h in a protein-synthesis-independent manner. These results, together with other experiments, demonstrate that [1] the insulin signal is independent of PKC, [2] insulin acts as a weak competence and a strong progression factor, while TPA behaves as a strong competence factor, and [3] the 9-10-h lag is made up of a 3-h period which is independent of protein synthesis, advancing the cells to a post-G(o) state of 'competence'.