c-myc is an important protooncogene whose misregulation is believed to causally affect the development of numerous human cancers. c-myc null rat fibroblasts are viable but display a severe (two- to threefold) retardation of proliferation. The rates of RNA and protein synthesis are reduced by approximately the same factor, whereas cell size remains unaffected. We have performed a detailed kinetic cell cycle analysis of c-myc ^−/− cells by using several labeling and synchronization methods. The majority of cells (>90%) in asynchronous, exponential phase c-myc ^−/−cultures cycle continuously with uniformly elongated cell cycles. Cell cycle elongation is due to a major lengthening of G₁ phase (four- to fivefold) and a more limited lengthening of G₂phase (twofold), whereas S phase duration is largely unaffected. Progression from mitosis to the G1 restriction point and the subsequent progression from the restriction point into S phase are both drastically delayed. These results are best explained by a model in which c-Myc directly affects cell growth (accumulation of mass) and cell proliferation (the cell cycle machinery) by independent pathways.