The properties of the blood-brain barrier are those of the capillary endothelium in brain. This endothelium contrasts with that elsewhere in being sealed with tight junctions, having a high electrical resistance and low permeability to polar solutes. It is exceptional in having a paucity of pits and vesicles, a specialised enzyme content and a high density of mitochondria. Functionally, a range of transport mechanisms allow rapid movement of certain specific metabolic substrates. Ion pumps are concerned with secretion of brain interstitial fluid and regulation of its ionic concentration. The retinal capillaries are largely identical to those of the brain, but entry of solutes into retina is also determined by the properties of the pigment epithelium, functionally separating the retina from the highly vascular choroid. A clear difference lies in the greater resistance of cerebral microvessels to diabetic damage. The mechanism of this difference is unclear, but may relate to a better control of the brain interstitial fluid at a lower glucose concentration than is possible in the retinal interstitial fluid.