The functional ultrastructure of the human arachnoid villi was studied to clarify drainage channels of cerebrospinal fluid (CSF). The apical portion of each villus was usually covered by the arachnoid cell layer alone with no endothelial investment, whereas most of the stromal central core was further encompassed by a fibrous capsule with an endothelial investment. Accordingly, the CSF-blood interface was assumed to be in both the endothelial cells and the arachnoid cell layer. The former were characterized by abundant micropinocytotic vesicles and occasional intracytoplasmic vacuoles, whereas the latter was characterized by numerous extracellular cisterns measuring 10 micron in maximal diameter. There were no free communications such as endothelial open junctions or endothelium-lined tubules. In the villi affected by subarachnoid hemorrhage, endothelial cells were intact and continuous despite the erythrocyte-packed subendothelial space, which appeared to be on the verge of rupturing. Intracytoplasmic vacuoles, measuring less than 1 micron diameter, sometimes contained serum protein-like substance. Furthermore, the extracellular cisterns were distended by intact or disintegrating erythrocytes that served as a natural tracer, suggesting CSF drainage channels. It is conceivable that, in human arachnoid villi, the extracellular cisterns of the arachnoid cell layer contribute to the passive transport of CSF, whereas micropinocytosis and vacuolization mechanisms of the endothelial cells are available for active transport.