Somatostatin (SS) is found in the endocrine pancreas and has been reported in the pure pancreatic juice (PPJ) of different species. Characterization by gel filtration of immunoreactive SS (irSS) in the rat PPJ (rPPJ) results in a single peak corresponding to 23kDa molecular weight. Incubation of the 23kDa fraction with labeled or synthetic SS results in time dependent degradation of both peptides. This degradation is inhibitable by PMSF, calcium and by heat, whereas specific inhibitors of trypsin and chymotrypsin are without effect. These data suggest that irSS previously measured in rPPJ samples by RIA without confirmation of radioactive tracer stability may lead to false positive results. Indeed, our study indicates the presence of a 23kDa enzyme in the rPPJ degrading radiolabeled somatostatin during the RIA procedure. This putative new enzyme found into the rPPJ may thus be partially responsible for the apparent irSS presence.