The vocal control nucleus of the adult songbird forebrain, HVc, exhibits de novo neurogenesis in adulthood, with the production of new neurons from precursor cells located in the overlying ventricular zone (Goldman and Nottebohm, 1983). We previously established that explants derived from neurogenic regions of the adult canary forebrain could be maintained in vitro, under conditions that permitted the migration and differentiation of those new neurons previously generated in vivo (Goldman, 1990). However, we found no evidence for continued neuronal mitogenesis in these cultures, which were raised in high concentrations of serum. In the present study, we investigated the permissive conditions for in vitro neurogenesis by these adult forebrain-derived ventricular zone explants. When HVc explants derived from adult zebra finches were maintained in low-serum medium, in vitro neurogenesis could be demonstrated by 3H-thymidine uptake as long as 5 days after explantation. Immunocytochemistry for microtubule-associated protein-2 followed by autoradiography confirmed the neuronal identity of these 3H- thymidine-incorporating cells. In vitro neuronal production occurred in an inverse relation to the serum concentration: Over the range of 2.5– 25% fetal bovine serum, neuronal 3H-thymidine labeling was most frequent in those cultures exposed to the lowest serum levels. This facilitation of in vitro neuronal mitogenesis by serum depletion suggests that fetal serum may contain factors that inhibit the division of adult-derived neuronal precursor cells, either directly or by agents released by serum-stimulated glial or ependymal cells.