Insulin-like growth factor I (IGF-I) is the mediator of growth hormone dependent growth. The peptide has been identified by radioimmunoassay in a number of human exocrine secretions of the gastrointestinal tract including (nM): saliva 0.9, gastric juice 3.5, jejunal chyme 24.6, pancreatic juice 3.6, and bile 0.9. The identification of IGF-I in pancreatic juice was confirmed by HPLC. The intravenous injection of 1 unit/kg secretin increased pancreatic juice IGF-I content from a basal level of roughly 4 nM to nearly 20 nM. Conversely, the IGF-I content of bile was unaffected by secretin. Radioligand blot analysis of samples of gastric juice, jejunal chyme and pancreatic juice demonstrated that these fluids contained no IGF binding proteins. Thus, unlike IGF-I in serum, IGF-I secreted into the gastrointestinal lumen is not bound to insulin-like growth factor I binding proteins. Since the growth factor is not protein bound, its concentration in the gut lumen may be high enough to exert biological activity.