Cholecystokinin (CCK) is a known stimulus for the release of insulin and other islet hormones. To localize islet cell CCK binding sites, we measured the uptake of 125I-CCK by the isolated, perfused rat pancreas. Light microscope autoradiographs revealed uptake of label over both the endocrine islets of Langerhans and the exocrine acini. This uptake of 125I-CCK was saturable, as it decreased markedly when a large excess of unlabeled CCK8 was included in the perfusion solution. To define which cells in the islets bound CCK, electron microscope autoradiographs were prepared. The majority of silver grains in islets were localized over beta cells (69%), although saturable uptake was also observed over alpha (12%) and other islet cells. When grain densities were analyzed (grains/μm2), the highest density was observed over islet blood vessel cells. In contrast to islet blood vessels, there was no localization of 125I-CCK over acinar blood vessels. This study supports the concept, therefore, that there is a direct regulation of islet endocrine cells by CCK, and also raises the possibility that CCK influences islet hormone release via an indirect effect on the islet vascular endothelium.