The vasculature of the islets of Langerhans was studied in rats using methacrylate corrosion casts and islet reconstructions from stained serial paraffin sections. In corrosion casts, which allowed a three-dimensional view of the pancreatic vasculature, all islets had one or two afferent arterioles, which gave off numerous capillaries to form a glomerular-like network. Islets could be grouped in three classes on the basis of size. Moreover, these classes had preferential locations within the vascular tree: the smaller the islet, the more peripheral. In small islets (those less than 160 micrometers in diameter) efferent capillaries arose from this network and either coalesced at the periphery of the islet or passed through perinsular exocrine tissue before coalescing into venules. However, in intermediate islets (those 160--260 micrometers in diameter) and large islets (those greater than 260 micrometers in diameter) efferent capillaries usually coalesced at the edge of the islet forming an extensive fingerlike network of collecting venules over the islet. This suggested that at least in the rat a large amount of the islet tissue is directly drained by venules. In serial paraffin sections of islets perfused with India ink and stained alternately for B-cells or for non-B-cells, the relation of the blood vessels and the organized array of different cell types making up the islet was discernible. In islets of all sizes, the afferent arterioles entered the islet of all sizes, the afferent arterioles entered the islet at discontinuities of the mantle of non-B-(glucagon, somatostatin, and pancreatic polypeptide) cells. Entering at the B-cell mass, the arterioles broke into capillaries that traversed the B-cell core before passing through the opposite non-B-cell mantle. The afferent capillaries coalesced into collecting venules outside the islet. In intermediate and large islets, the overlying collecting venule network was closely apposed to the mantle. These anatomic findings indicate that in the rat islet only some of the efferent vessels are part of a insuloacinar portal system and that the afferent vessels reach the B-cell core without passing through the non-B-cell islet tissue.