Neurons destined for the cerebral neocortex are formed in the pseudostratified ventricular epithelium (PVE) lining the ventricular cavity of the developing cerebral wall. The present study, based upon cumulative S-phase labeling with bromodeoxyuridine, is an analysis of cell cycle parameters of the PVE. It is undertaken in the dorsomedial cerebral wall of mouse embryos from the eleventh to the seventeenth gestational day (E11-E17, day of conception = E0) corresponding to the complete period of neuronogenesis. The growth fraction (fraction of cells in the population which is proliferating) is virtually 1.0 from E11 through E16. The length of the cell cycle increases from 8.1 to 18.4 hr, which corresponds to a sequence of 11 integer cell cycles over the course of neuronal cytogenesis in mice. The increase in the length of the cell cycle is due essentially to a fourfold increase in the length of G1 phase which is the only phase of the cell cycle which varies systematically. Thus, the G1 phase is most likely to be the phase of the cell cycle which is modulated by extrinsically and intrinsically acting mechanisms involved in the regulation of neuronal cytogenesis.