An increasing body of evidence implies that the results of pharmacological studies of blood vessels may depend on the presence of an intact layer of endothelial cells inside the blood vessels. This is often inadvertently removed during the early part of the experiment and it is thus necessary to have some means of determining whether the endothelium is intact or has suffered extensive damage. Previous reports describe a stain that will enable the endothelial cells to be visualized, however we find that in the cerebral vasculature this approach is unsatisfactory in that it provides a gross underestimate of the amount of intact endothelium. A modification of the original procedure is described that provides results that correspond well with those of scanning or transmission electron microscopy.