By intra-arterial postmortem staining of the pancreas with hematoxylin after the administration of nonradioactive microspheres to anesthetized unfasted rats, the following values (+/-SE) were obtained: mean single islet volume, 1.00 +/- 0.12 nl (median 0.32 +/- 0.04 nl, n = 14); pancreatic intensity of perfusion, 1.18 +/- 0.14 ml X min-1 X g-1 (n = 10); percentage of pancreatic flow to islets, 6 +/- 1% (n = 10); single islet blood flow, 20 +/- 3 ml X min-1 (n = 10); and islet perfusion, 19 +/- 3 ml X min-1 X g-1 (n = 10). Perfusion of islet tissue was calculated to be above average for very small islets and to decrease with increasing islet size to become below average for very large ones. Details of the distribution of the total islet flow to subpopulations of islets grouped according to single islet size are shown pictorially.