In order to determine the precise localization of somatostatin (SRIF) in the pancreas, electron microscopic immunocytochemistry was performed on thin sections of whole pancreas and isolated pancreatic islets of the rat using the peroxidase anti-peroxidase technique. SRIF was localized in secretory granules concentrated in cells sparsely distributed near the periphery of the islet. The granules were closely applied to their limiting membrane, exhibited moderate electron density, and were smaller than those in other islet cells. The location and relative number of SRIF-containing cells as well as the morphology of the granules suggest that SRIF is present in delta cells or a subgroup with small granules. These results provide evidence that SRIF is present in a discrete granule population in a specific type of secretory cell in the pancreatic islets.