The adult mammalian forebrain contains a population of multipotential neural stem cells in the subependyma of the lateral ventricles whose progeny are the constitutively proliferating cells, which divide actively throughout life. The adult mammalian brain is ideal for examining the kinetics of the stem cells due to their strict spatial localization and the limited and discrete type of progeny generated (constitutively proliferating cells). Clonal lineage analyses 6 days after retrovirus infection revealed that under baseline conditions 60% of the constitutively proliferating cells undergo cell death, 25% migrate to the olfactory bulb and 15% remain confined to the lateral ventricle subependyma (where they reside for approximately 15 days). Analysis of single cell clones 31 days after retroviral infection revealed that the stem cell divides asymmetrically to self-renew and give rise to constitutively proliferating cells. Following repopulation of the depleted subependyma the average clone size is 2.8 times larger than control, yet the absolute number of cells migrating to the olfactory bulb is maintained and the stem cell retains its asymmetric mode of division. The number of neural stem cells in the adult forebrain 33 days after repopulation of the subependyma was estimated using bromodeoxyuridine labeling of subepenydmal cells. There were calculated to be 1200–1300 cells between the rostral corpus callosum and rostral anterior commissure; these data support a lineage model similar to those based on stem cell behavior in other tissue types.