Insulin-like growth factor I (IGF-I) and transforming growth factor-1 (TGF-1) are upregulated in myofibroblasts at sites of fibrosis in experimental enterocolitis and in Crohn's disease (CD). We compared the sites of expression of IGF-I and TGF-1 in a rat peptidoglycan-polysaccharide (PG-PS) model of chronic granulomatous enterocolitis and fibrosis. We used the human colonic CCD-18Co fibroblast/myofibroblast cell line to test the hypothesis that TGF-1 and IGF-I interact to regulate proliferation, collagen synthesis, and activated phenotype typified by expression of -smooth muscle actin and organization into stress fibers. IGF-I potently stimulated while TGF-1 inhibited basal DNA synthesis. TGF-1 and IGF-I each had similar but not additive effects to induce type I collagen. TGF-1 but not IGF-I potently stimulated expression of -smooth muscle actin and stress fiber formation. IGF-I in combination with TGF-1 attenuated stress fiber formation without reducing -smooth muscle actin expression. Stress fibers were not a prerequisite for increased collagen synthesis. TGF-1 upregulated IGF-I mRNA, which led us to examine the effects of IGF-I in cells previously activated by TGF-1 pretreatment. IGF-I potently stimulated proliferation of TGF-1-activated myofibroblasts without reversing activated fibrogenic phenotype. We conclude that TGF-1 and IGF-I both stimulate type I collagen synthesis but have differential effects on activated phenotype and proliferation. We propose that during intestinal inflammation, regulation of activated phenotype and proliferation may require sequential actions of TGF-1 and IGF-I, but they may act in concert to increase collagen deposition.