The ultrastructural features that are critical to the efficient functioning of the blood-brain barrier (b-bb) were quantified in typical barrier vessels, i.e., cerebral gray and white matter capillaries, and compared with the same parameters in cerebral nonbarrier capillaries (area postrema), and in somatic capillaries, (muscle) of the mouse. The image analysis was done with the help of a simple, rapid system that required only a digitized bit pad and a microcomputer. We found that: (1) The number of pinocytotic vesicles that are thought to be associated with vascular permeability was three times higher in area postrema vessels and nearly seven times higher in muscle vessels than in barrier vessels. (2) A percentage of interendothelial junctions in nonbarrier capillaries displayed areas of separation that may represent interendothelial channels coursing from the luminal to the abluminal surface between areas of tight junctional complexes. Such areas were not observed in barrier capillaries under the normal conditions studied here, but have been seen by others under conditions in which the barrier has been breached. (3) A 39% decrease in wall thickness in barrier capillaries. (4) No differences in mitochondrial density or in area of associated pericytes among capillaries from any region. Therefore we have questioned both the universality of the apparently increased metabolic work capacity of barrier capillaries, and whether pericytes play any role in the barrier under normal conditions.