In mice, the striatal compartment of the forebrain is established by embryonic day 11 (E11, E0 = day of conception) when a lateral ganglionic eminence emerges surrounding the lateral and ventral margins of the forebrain ventricles. The inception of the striatal compartment is evidence of altered cell cycle kinetics, especially a rapid production of postmitotic cells, within a discrete portion of the telencephalic neuroepithelium. As a step toward understanding the mechanisms which contribute to the development of a cytokinetically distinct striatal compartment, we characterized the rate and pattern of cellular output in the lateral ganglionic neuroepithelium of mice on E11. The data show that the striatal compartment is distinguished by concurrent and equivalent levels of cell output from two proliferative populations: a dominant secondary proliferative population and a smaller, pseudostratified ventricular epithelium. In addition, although the ganglionic neuroepithelium is expanding on E11, 30-35% of the daughter cells produced leave the cell cycle and become postmitotic. These cytogenetic events, occurring in the lateral ganglionic progenitor population, may contribute to the development of a distinct striatal compartment within the telencephalon.