A new method is described for morphological studies of blood vessels and related cellular elements in the mammalian central nervous system (CNS). The tracer protein, horseradish peroxidase (HRP), in solution, is infused intraventricularly or intracisternally in anesthetized animals over 5-10 min. During this period, HRP in the subarachnoid space enters the perivascular spaces around penetrating arterioles and rapidly permeates the gliovascular basal laminae surrounding capillaries. After fixation by intravascular perfusion of aldehydes, brain sections are incubated with the highly sensitive chromogen, tetramethylbenzidine. Intraparenchymal blood vessels throughout the CNS are vividly demonstrated for light microscopy by HRP reaction product in their perivascular spaces or basal laminae. Correlative ultrastructural investigations of specific blood vessels and related parenchymal elements can be conducted using adjacent sections.