Microcirculation in rat pancreas was studied by scanning electron microscopy of resin vascular casts and by light microscopy of India ink-injected and sectioned tissue samples. In the scanning survey of the casts, islets larger than 30 microns in diameter counted no less than 400 in the adult rat pancreas. They were located either in the exocrine lobules (intralobular islets, counting 210 or more), or in the interlobular tissue spaces and along the secretory ducts (extralobular islets, 190 or more). Every islet received arterial blood via one or more proper arterioles. These afferent vessels first divided in the peripheral zone of the islet consisting of A and D cells and later extended deeper to form a capillary network in the islet core consisting of B cells. Blood originating from this capillary network left the islet via efferent vessels. This microvascular pattern may favor the regulation by A and D cells over the secretory activity of B cells. The intralobular islets gave their efferent vessels to the capillary bed of the exocrine tissue of the lobule, thus forming an insulo-acinar portal system. When the lobule was relatively small in size, superfluent blood was led directly to veins via insulo-venous drainage vessels. The intralobular islets sometimes issued one or more long translobular portal vessels reaching adjacent lobules. The extralobular islets issued either insulovenous vessels draining to interlobular veins, or extralobular insulo-acinar portal vessels supplying adjacent lobules.