Experiments were designed to examine whether or not insulin-like growth factor I (IGF-I), which is produced by vascular cells in response to injury, affects the production of nitric oxide evoked by the inducible nitric oxide synthase in cultures of smooth muscle cells from the rat aorta. Nitric oxide production was assessed indirectly by the measurement of nitrite accumulation and nitric oxide synthase activity by determining the formation of L–citrulline from L–arginine. Nitric oxide synthase was induced in vascular smooth muscle cells that had been exposed to interleukin-1β (IL-1β) or tumor necrosis factor-α (TNF-α). IGF-I inhibited, in a concentration-dependent manner, the production of nitrite and L–citrulline evoked by IL-1β or TNF-α. The inhibition caused by IGF-I required the presence of the growth factor during the induction of nitric oxide synthase. Two IGF-I-related proteins, IGF-II and insulin, also inhibited, but to a smaller extent, the release of nitrite and the formation of L–citrulline stimulated by IL-1β. Under bioassay conditions, the perfusates from columns containing IL-1β-treated smooth muscle cells relaxed rings of rat aorta without endothelium that had been contracted with phenyl ephrine; these relaxations were reversed by nitro-L–arginine. Addition of IL-1β-treated vascular smooth muscle cells to indomethacin-treated platelets inhibited their aggregation to thrombin; methylene blue prevented this inhibition. Control smooth muscle cells or cells exposed to IGF-I alone did not have such effects. Smooth muscle cells that had been exposed simultaneously to IL-1β and IGF-I also relaxed detector rat aortic rings, but to a smaller extent, and minimally affected the aggregation. IL-1β caused the expression of inducible nitric oxide synthase mRNA levels in vascular smooth muscle cells; this response was reduced in cells treated with IL-1β in combination with IGF-I. These observations indicate that IGF-I inhibits the cytokine-induced production of nitric oxide by preventing the induction of nitric oxide synthase. They further suggest that IGF-I may be an important modulator of the production of nitric oxide at sites of vascular injury.