Nonradioactive microspheres of various sized (mean batch diameters ranging from 6 microns ato 26 microns) were administered to unfasted rabbits under sodium pentobarbital anesthesia by a brief injection into the left ventricle. Flow rate per bead was determined by the reference organ method. After prompt death of the animal, the spheres were located and counted miroscopically (islet vs. nonislet) in fixed, stained, and cleared portions of the pancreas. According to an analysis of the distribution of spheres as a function of bead diameter, 11-23% of the total pancreatic blood flow went directly to the islets and 77-89% to the "acini" (nonislet tissues). After retrograde postmortem injection of spheres 6 microns, 9 microns and 11 microns in diameter, practically none reached the islets, whereas after orthograde postmortem injection, they did so in the same proportions as in vivo. These results, supplemented by certain control experiments, support the view that all, or nearly all, efferent islet blood flow goes to the acinar capillaries before leaving the organ. We conclude that the arterial supplies to the rabbit exocrine and endocrine pancreas are in parallel, with most of the flow going to the exocrine portion. However, the flow to the islets is large enough to permit significant local actions of the islet hormones on the exocrine pancreas, in confirmation of the existence of an insuloacinar portal system.