We have shown successful in vitro expansion of rodent and human neural precursor cells (NPC) derived from fetal midbrain and forebrain. Here, we show that mouse neural precursor cells growing in neurospheres proliferate, but also undergo spontaneous apoptosis in vitro. On average, 30.7 +/- 3.4% cells of midbrain-derived neural precursors and 32.1 +/- 2.5% of forebrain-derived neural precursors were found apoptotic within neurospheres. Spontaneous apoptosis involved mitochondrial cytochrome c release and activation of effector caspase-3. Caspase-3 was activated in 26.9 +/- 3.4% of mesencephalic neural precursor cells. Virtually all nuclei with morphological signs of apoptosis belong to caspase-3-positive cells. The great majority of dying cells within neurospheres was positive for CNS precursor cell marker nestin. Pro-apoptotic proteins of the Bcl-2 family, Bax and Bak, exhibited conformational changes in neural precursors expanding in vitro. Key molecules such as executioner caspase-3 may be useful targets to reduce the amount of apoptosis.