In passing from islets to the exocrine part of the gland, pancreatic capillaries change their character. Islet capillaries are significantly wider than exocrine (5.27 micrometers, diameter compared with 4.35 micrometers), thinner walled and have many more fenestrations micrometers-1 of endothelium (1.3 compared with 0.13). The point of transition at the edge of the islet is very abrupt, for capillaries that are in contact with both endocrine and exocrine tissue ('edge capillaries') have significantly more fenestrae on the side facing the endocrine cells than the side facing exocrine (1.46 fenestrae micrometers-1 compared with 0.3 micrometers-1). The structure of these edge capillaries suggests that the factors operating to induce the formation of fenestrae do so over extremely short distances. The relationship between fenestrations, the high blood flow and high water flux seen in endocrine glands is discussed.