Cells of the corpus striatum arise from the lateral ganglionic eminence of the telencephalic neuroepithelium. In mice, the length of the cell cycle and that of its component phases were estimated, and the interkinetic nuclear migratory pattern was characterized for lateral ganglionic progenitors on embryonic day 11 and 12 to gain insights into striatal cytogenetic process. An S-phase labeling paradigm using bromodeoxyuridine was employed. To assess regional variation in proliferative patterns, rostral, middle and caudal levels of the ganglionic neuroepithelium were examined separately. The pattern of interkinetic nuclear migration and the duration of G1-, G2-, and M-phases at the rostral and middle levels differed from those at the caudal level. The length of the cell cycle and that of the G1-phase increased during the interval embryonic day 11 to 12, especially at the rostral and middle levels. During the same interval, a sizable secondary proliferative population emerged at all three levels. Two principal conclusions are drawn: Progenitors at the different rostrocaudal levels are heterogeneous with regard to the pattern of cellular proliferation, and ganglionic progenitors are in advance of the cerebral cortical progenitors based on the relative size of the secondary proliferative population and the magnitude of cytokinetic parameters. Thus, proliferative behavior distinguishes progenitor populations at different rostrocaudal levels within the lateral ganglionic neuroepithelium and across the ganglionic and cerebral cortical domains of the telencephalic neuroepithelium.