This study was undertaken to determine the origin and forms of immunoreactive somatostatin (SLI) present in the pancreatic juice of cannulated nonanesthetized rats. The systemic origin of somatostatin (SS) was assessed from bolus intravenous injections of either I125-Tyr1-SS-14 or synthetic SS-14 under basal and stimulated pancreatic juice secretion, and from measurements of radioactivity and SS-14 in the collected juice. Gel filtration of pooled juice samples indicated that there were no forms of SS corresponding to standards of I125-Tyr1-SS-14 or SS-14, but radioactive material eluted in the area of either I125-Tyr or free iodine. Investigation of the presence of SS-14 in the juice by radioimmunoassay (RIA) led to the identification of a 23-kDa SS-like immunoreactive protein that did not correspond to any of the three known forms of SS present in the pancreatic tissue (Pro-SS, SS-28, and SS-14). By boiling the juice before the RIA, this 23-kDa protein was no longer detectable by RIA. These data suggest that this protein may be a "putative" enzyme, degrading the SS tracer in the RIA. This possibility is supported by the observation that boiled juice lost this capacity of interfering with the RIA. In conclusion, SS in the pancreatic juice, if present, is not from systemic origin and the juice contains an active thermosensitive substance degrading the SS tracer in the RIA.