Rat cerebral cortex synaptosomes were exposed in superfusion to various depolarizing stimuli and the release of somatostatin-like immunoreactivity (SRIF-LI) was measured by means of a radioimmunoassay procedure. High KCl (9-50 mM) concentration dependently evoked SRIF-LI release; the evoked overflow reached a plateau at 25 mM KCl and was completely abolished when Ca2+ ions were omitted from the superfusion medium, independently of the concentration of KCl used. The 15 mM K(+)-evoked release of SRIF-LI increased sharply as the Ca2+ concentration was raised to 0.8 mM, then leveled off and reached a plateau at 1.2 mM. The 15 mM K(+)-evoked overflow, but not the spontaneous outflow, was partially decreased (50%) by 1 microM tetrodotoxin. The presence in the superfusion fluid of a mixture of peptidase inhibitors did not improve the recovery of SRIF-LI both in the absence and in the presence of high K+. Exposure of synaptosomes to veratrine (1-50 microM) induced release of SRIF-LI in a concentration-dependent way. The effect of the alkaloid was strictly Ca2+ and tetrodotoxin sensitive. Replacement of extracellular Na+ by sucrose caused an acceleration of the spontaneous SRIF-LI outflow that was inversely correlated to the Na+ content in the superfusion medium. The release evoked by the sodium-deprived media did not exhibit any calcium dependence. HPLC analysis of the samples collected during superfusion showed that greater than 90% of the SRIF-LI released either during the spontaneous outflow or by 15 mM KCl was represented by SRIF-14 (SRIF-28(14-28]. These values reflected the ratio SRIF-14/SRIF-28 found in synaptosomes at the end of the experiments.