Kinetic analysis of cellular response to serum deprivation or inhibition of protein synthesis was performed on Swiss 3T3 cells. Time-lapse cinematographic analysis of individual cells transiently exposed to serum-free medium (with or without the addition of purified growth factors) or cycloheximide enabled a detailed mapping of the magnitude and variability of cellular response in different parts of the cell cycle. In all cells, in all stages of the cell cycle, serum deprivation resulted in inhibition of protein synthesis, but only in postmitotic cells in the first 3-4 hr of G1 (here denoted the G1pm phase) did it produce cell-cycle arrest. During G1pm, the cells are highly dependent on the continuous presence of serum growth factors and a high level of protein synthesis in order to progress toward mitosis. A 1-hr exposure to serum-free medium or to cycloheximide was sufficient to force most G1pm cells into a state of quiescence (G0), from which the cells required 8 hr to return to G1pm. During G1pm the cells complete the growth factor-dependent processes leading to commitment for proliferation. Thereafter they enter the growth factor-independent pre-DNA-synthetic part of G1 (here denoted G1ps). The commitment process in G1pm could be successfully completed in the presence of platelet-derived growth factor as the only supplied growth factor. Epidermal growth factor and insulin were insufficient for the completion of the commitment processes in G1pm, although they were able to temporarily prevent the G1pm cells from entering G0 during serum starvation. Under conditions optimal for proliferation, the cells complete the commitment processes in G1pm within a remarkably constant time period. Almost all cells in the population left G1pm and entered G1ps between the third and fourth hour after mitosis. The duration of G1ps, on the other hand, showed a large intercellular variability consistent with a transition-probability event. In fact, G1ps accounts for most of the variability in G1 and cell cycle time.