The effects of ethanol on the cell cycle kinetics of cortical precursor cells during the period of cortical neuronogenesis [between gestational day (G) 12 and G21] was systematically examined. Samples of dissociated cortical cells were harvested from the cerebral cortices of 13-, 15-, 17-, 19-, and 21-day-old fetuses. The fetuses were obtained from pregnant rats: (a) fed a liquid diet containing 6.7% (v/v) ethanol (Et) ad libitum, (b) pair-fed an isocaloric liquid control diet (Ct), or (c) fed chow and water (Ch) ad libitum. Before harvesting the cells, the fetuses were administered a series of 1-5 injections of bromodeoxyuridine (BrdU). The proportion of cells that incorporated the BrdU was assessed. Using these raw data, the S-phase length (T-s, total cell cycle length (T-c, and the growth fraction (GF; the fraction of the total population that was actively cycling) were determined with a cumulative labeling procedure. The T-s was similar to 8-9 hr, regardless of either the date of the injection or the dietary treatment of the dam. On the other hand, the T-c for the Ct- and Ch-treated rats increased over the gestational period. That is, the Tc was shortest on G13 and longest on G21. The T-c for Et-treated rats, however, did not change between G13 and G21. For the Ch- and Ct-treated groups, the GF decreased >15-fold between G13 and G21. The decline (5-fold) for the Et-treated group over the same period, however, was not as great as it was for the Ct-treated fetuses. Thus, by G17 (and thereafter), the GF for Et-treated fetuses was significantly greater than it was for the Ct-treated group. Ethanol treatment has opposite effects on the two cortical germinal zones; it stimulates the proliferation of subventricular cells, whereas it inhibits the proliferation of ventricular cells). Thus, the ethanol-induced changes in the T-c and the GF reflect the combined effects of ethanol on the two cortical proliferative zones.