In most areas of the body, arteries and veins run close together, often sharing a common connective tissue sheath. One exception to this is observed in the brain, where arteries come in from the base and veins collect over the convexity. Classically the larger blood vessels are formed by three coats: intima, media, and adventitia. Leptomeningeal vessels are further reinforced by a monolayer of pial cells. In the guinea pig, however, above the corpus callosum we found a group of blood vessels (an artery and several veins) enclosed in a common leptomeningeal sheath. The artery arises at the confluence of the anterior cerebral arteries; the veins drain into the straight sinus. The epithelial nature of the sheath is evident by the close apposition of cell membranes, the presence of junctional devices, and the existence of a basal lamina. The ultrastructural features of this epithelium are similar to those of the arachnoid-dural membrane. Whether this peculiar vascular complex has any specific function needs to be investigated further. The presence of these vessels apparently 'isolated' within a leptomeningeal subcompartment may provide a suitable model to study vascular-extravascular-cerebrospinal fluid substance exchange.