To determine whether insulin receptor substrate-1 (IRS-1) is essential in mediating insulin-like growth factor-I (IGF-I) stimulation of brain growth and myelination in vivo, we cross-bred IGF-I transgenic (Tg) mice with IRS-1 null mutant (IRS-1(-/-)) mice and examined brain growth and expression of myelin-specific proteins in mice that overexpress IGF-I with or without IRS-1 expression. We found that while IGF-I overexpression stimulates a dramatic increase in brain weight (43%) by 7-8 weeks of age in the absence of IRS-1, it stimulates a greater increase (50%) with intact IRS-1 expression. To evaluate myelination we investigated IGF-I-stimulated expression of myelin basic protein (MBP) and proteolipid protein (PLP) in the cerebral cortex CTX and brainstem, and found similar increases in each region in IRS-1(-/-) and wild type mice. In studies using mixed glial cultures derived from IRS-1(-/-) mice, IGF-I also increased the abundance of MBP and PLP mRNA. To assess possible alternate mediators of IGF-I actions, we examined IRS-2 and IRS-4 and found that the abundance of each is increased in the CTX of IRS-1(-/-) mice and IGF-I Tg mice. Our results suggest that IRS-1 is not essential in IGF-I promotion of oligodendrocyte development and myelination, and that IRS-2 and IRS-4 may compensate for the loss of IRS-1 expression and function in the cells of oligodendrocyte lineage. Nonetheless, the finding that IGF-I stimulates brain growth less well in the absence of IRS-1 suggests that IRS-1-mediated signaling may be more central to IGF-I action in cells other than glia and oligodendrocytes.