Upregulation of the proto-oncoprotein Myc, a basic, helix-loop-helix, leucin zipper domain transcription factor has profound consequences on cell proliferation, cell growth and apoptosis. Cell cultures of somatic c-myc-/- rat fibroblasts show extremely prolonged doubling times of 52 h. Using time-lapse microscopy, we show here that individual c-myc-/- cells proceeded within approximately 24 h through the cell cycle as fast as c-myc+/+ cells. However, c-myc-/- cells were highly sensitive to contact inhibition and readily arrested in the cell cycle already at low density. Activation of conditional MycER overcame cell cycle arrest in c-myc-/- cells and led to continuous proliferation at the expense of increased apoptosis at high cell density. Conditional expression of Mad1, a Myc antagonist, represses proliferation of different cell types including U2OS cells. In analogy to the effect of Myc, this occurs mainly by reducing the probability of cells remaining in the cycle. Our data demonstrate that the Myc/Max/Mad network does not regulate the duration of the cell cycle, but the decision of cells to enter or exit the cell cycle.