Nestin is a protein that is thought to be expressed in neural stem cells; however, there is a paucity of data on nestin expression in vivo, and little is known of the relationship between nestin and mitotically active cell populations in the subventricular zones (SVZ). In this study, the subventricular zone of the third ventricle contained a high proportion of cells that expressed nestin, while there were significantly fewer cells that expressed nestin in the SVZ of the lateral ventricles. In contrast, bromodeoxyuridine (BrdU) immunoreactivity was the diametric opposite, being higher in the SVZ of the lateral ventricle than in the SVZ of the third ventricle. Morphological and anatomical evidence suggests that nestin-expressing cells in these two areas may be different cell types. In a separate set of experiments, an acute localized lesion was induced adjacent to one of the ventricles. While the number of BrdU cells and Ki-67 cells in the SVZs increased with this manipulation, the number of nestin-expressing cells did not change significantly. These data indicate that the expression of nestin does not correlate with mitotic activity in cells of the SVZs under either normal or inflammatory conditions. It is hypothesized that nestin-expressing cells in the SVZs may give way to transit amplifying cells that in turn give way to immature neurons or glia. These transit-amplifying cells may have a much higher rate of mitosis than nestin-positive cells and may react to neural damage by increasing their rate of proliferation.