The 3-D relation between the lobules, terminal blood vessels and Langerhans islets of human pancreas was established by graphic reconstruction from serial histologic sections. The material included three pancreases obtained at autopsy. It was found that the lobule is an assembly of sublobular parenchymal units which have a terminal arteriole at the center. These, being considered to be an elemental structure of microcirculation, were named the "primary lobule" of the pancreas. The concept was also supported by the 3-D analysis of a pancreas "pseudolobulated" due to chronic venous congestion, in which "primary lobules" were clearly demarcated by atrophic zones distributed along the periphery of circulation. Four types of terminal arterioles were discriminated: 1) those terminating at an islet, 2) those having no relation with islet, 3) those with islets dispersed near the ending and 4) those coming directly from an interlobular artery. More than half the endings were found to belong to either Type 2 or 3, which was quite different from the traditional assumption of complete arterio-insular correlation. Consequently, islets could also be classified according to whether or not having an afferent arteriole, namely, into "arterial" and "non-arterial" islets. The "arterial" islets amounted to 75% in the total islet volume, although in islet number, the "non-arterial" ones accounted for as much as 72.5%. Thus, at least in man, functional correlation between islets and exocrine gland is not so tight as claimed by the insulo-acinar axis theory.