Insulin-like growth factor I (IGF-I) potently stimulates intestinal growth. Insulin receptor substrate 1 (IRS-1) mediates proliferative and anti-apoptotic actions of IGF-I in cell lines but its in vivo relevance in intestine is not defined. This study tested the hypothesis that there is cell type-specific dependence on IRS-1 as a mediator of IGF-I action. Length, mass, crypt cell proliferation and apoptosis were measured in small intestine and colon of IRS-1 null mice and wild type (WT) littermates and colon of IRS-1 null or WT mice expressing IGF-I transgenes. Expression of IGF-IR and signaling intermediates was examined in intestine of WT and IRS-1 null mice, cultured intestinal epithelial cells and myofibroblasts. Results: Absolute IRS-1 deficiency reduced mucosal mass in jejunum and colon but effects were more pronounced in colon. Muscularis mass was decreased in both segments. In IGF-I transgenics, IRS-1 deficiency significantly attenuated IGF-I-stimulated growth of colonic mucosa and abolished anti-apoptotic but not mitogenic effects of IGF-I transgene on crypt cells. IGF-I-induced muscularis growth was unaffected by IRS-1 deficiency. In intestinal epithelial cells, IRS-1 was expressed at higher levels than IRS-2 and was preferentially activated by IGF-I. In contrast, IGF-I activated both IRS-1 and IRS-2 in intestinal myofibroblasts and IRS-2 activation was up-regulated in IRS-1 null myofibroblasts. We conclude that the intestinal epithelium but not the muscularis requires IRS-1 for normal trophic actions of IGF-I, and that IRS-1 is required for anti-apoptotic but not the mitogenic effects of IGF-I in the intestinal crypts in vivo.