Methacrylate casts of the pancreatic blood vessels of the monkey were observed under the scanning electron microscope. Microdissection and scanning of the casts were repeated in order to examine the deeper parts of interest successively. It was demonstrated that a vas afferens, a branch of the intralobular artery, enters deep into the islet which is represented by a three-dimensional net of irregularly swollen capillaries. The termination of the vas afferens at the center of the islet in the monkey was discussed in relation to the central position of A cells in this animal. From the periphery of the islet numerous vasa efferentia radiate into the exocrine tissue to be connected, at a certain distance, with the capillary network of the latter. They are refered to as insulo-acinar portal vessels as they convey the islet hormones to their first target tissue, the exocrine pancreas.