Permeability of intracranial extracerebral arteries of stroke-prone spontaneously hypertensive rats (SHRSP) was studied using labeling techniques (ferritin and horseradish peroxidase), at the cellular level. In the arterial endothelial cells, the tracer molecules were slowly but constantly transported by the plasmalemmal vesicles to the subendothelial space. This endothelial transportation of the tracers into these cerebral arteries did not seem to be significantly influenced by aging, increased blood pressure, hyperlipidemia or the existence of cerebral bleeding and infarction. Around the adventitia, there were a great number of periadventitial capillaries, especially near bifurcations. In the periadventitial capillaries, the tracer molecules were readily trapped by endothelial cells and were quickly transported to pericapillary spaces. The tracer molecules were then detected in the phagocytes adjacent to the deeper layers of the media, and further in the medial smooth muscle cells. The possibility that large amounts of plasma components are supplied to the media from periadventitial capillaries in the intracranial extracerebral arteries has to be considered in the pathogenic mechanisms of cerebrovascular lesions.