The purpose of the present investigation was to determine whether endogenously produced interleukin (IL)-1 mediates the changes in insulin-like growth factor (IGF) I and IGF binding proteins (IGFBP) induced by chronic abdominal sepsis in rats and to correlate the changes in the IGF system with the alternations in protein synthesis. A constant infusion of IL-1 receptor antagonist (IL-1ra) was begun after the induction of sepsis and was continued for 5 days. Sepsis decreased IGF-I levels in the blood, liver, and gastrocnemius muscle, increased the content in the kidney, and did not alter IGF-I levels in heart, jejunum, and spleen. IL-1ra attenuated the sepsis-induced decrease in plasma IGF-I and completely prevented the changes in IGF-I observed in liver, kidney, and the gastrocnemius. IGFBP-1 was increased in the blood, liver, and muscle of septic rats. IL-1ra prevented this increase in IGFBP-1 in blood and liver but not in muscle. The rate of in vivo protein synthesis was decreased in the gastrocnemius and kidney and unaltered in the heart, liver, jejunum, and spleen. A strong linear correlation existed between levels of IGF-I and the rate of protein synthesis determined simultaneously in the gastrocnemius. These results provide evidence for the role of IL-1 as an endogenous mediator of the sepsis-induced changes in IGF-I and IGFBP-1 and suggest that the accompanying changes in muscle protein synthesis are partially mediated via changes in IGF-I.