The present report is an analysis of the proliferative behavior of the secondary proliferative population (SPP) of the dorsomedial region of the embryonic mouse cerebral wall. It is based upon experiments undertaken on embryonic days 14-16 (E14-E16) and exploits methods in which proliferative cells are labeled in S phase with either or both bromodeoxyuridine and tritiated thymidine. The SPP, which arises from the PVE by E13, is principally the progenitor population to the neuroglial population of the mature neocortex and subjacent cerebral wall. By the end of E14 the SPP comes to be distributed diffusely from the outer margin of the ventricular zone throughout subventricular zone and intermediate zone. The length of the cell cycle of the SPP is constant at approximately 15 hr throughout this interval; thus, this population undergoes 1.6 cell cycles/24 hr or 3.2 cycles in the course of the 48 hr period, E14-E16. Over this 48 hr period, the SPP increases from 11% to 35% of the total proliferative population of the dorsomedial cerebral wall. The absolute size of the SPP increases nearly sixfold. With these values taken together it may be estimated that approximately 87% of postmitotic cells of the SPP reenter S phase after each cell division in this interval which means that only approximately 13% of the proliferative population exits the cycle. These findings illustrate the massive expansion of the SPP antecedent to the explosive diffusion of glial cells through the neocortex and subjacent cerebral wall as neuronal migration comes to completion and neocortical growth and differentiation accelerate.