Intra-islet regulation of islet cells by one another is theoretically possible by two routes: (1) paracrine (i.e., interstitial, which is unproved); and (2) direct cellular perfusion through the islet microvasculature. The latter was tested in in vitro rat pancreases by anterograde and retrograde perfusion with or without anti-insulin or antisomatostatin antibody. Anterograde infusion of insulin antibody increased glucagon and somatostatin secretion (p less than 0.0005), whereas retrograde insulin antibody infusion was without effect. Anterograde infusion of somatostatin antibody had no effect upon insulin or glucagon secretion. In contrast, retrograde infusion of somatostatin antibody increased both insulin and glucagon secretion (p less than 0.0005). In comparison, anterograde infusion of antiglucagon antibody decreased somatostatin secretion without influencing insulin, whereas retrograde antiglucagon antibody infusion decreased insulin without changing somatostatin secretion. These results establish a "directed" functional microvascular circulation with a strict sequence of perfusion, first of B cells, then A cells, then D cells. The B cell microvascularly is the primary glucose sensor and its insulin plays a vital role in inhibiting glucagon secretion. The abnormalities in glucagon secretion in diabetes mellitus can now be explained by a deficiency in intra-islet microvascular insulin.