The in vivo actions of insulin-like growth factor-I (IGF-I) on prenatal and early postnatal brain development were investigated in transgenic (Tg) mice that overexpress IGF-I prenatally under the control of regulatory sequences from the nestin gene. Tg mice demonstrated increases in brain weight of 6% by embryonic day (E) 18 and 27% by postnatal day (P) 12. In Tg embryos at E16, the volume of the cortical plate was significantly increased by 52% and total cell number was increased by 54%. S-phase labeling with 5-bromo-2'-deoxyuridine revealed a 13-15% increase in the proportion of labeled neuroepithelial cells in Tg embryos at E14. In Tg mice at P12, significant increases in regional tissue volumes were detected in the cerebral cortex (29%), subcortical white matter (52%), caudate-putamen (37%), hippocampus (49%), dentate gyrus (71%) and habenular complex (48%). Tg mice exhibited significant increases in the total number of neurons in the cerebral cortex (27%), caudate-putamen (27%), dentate gyrus (69%), medial habenular nucleus (61%) and lateral habenular nucleus (36%). In the cerebral cortex and subcortical white matter of Tg mice, the total numbers of glial cells were significantly increased by 37% and 42%, respectively. The numerical density of apoptotic cells in the cerebral cortex, labeled by antibodies against active caspase-3, was reduced by 26% in Tg mice at P7. Our results demonstrate that IGF-I can both promote proliferation of neural cells in the embryonic central nervous system in vivo and inhibit their apoptosis during postnatal life.