Human arachnoid villi were studied ultrastructurally to clarify the mechanism of cerebrospinal fluid absorption. Arachnoid villi of humans showed quite different features from those of animals. The former were not always invested with endothelial linings as previously reported in the latter. Instead, there was a covering layer of arachnoid cells which consisted of both an electron-lucent outer zone and an electron-dense inner zone. The outer zone had less cytoplasmic filaments and desmosomes than the inner zone. The inner zone was basically indistinguishable from the arachnoid membrane and had numerous cytoplasmic filaments and a series of desmosomes. Often, the covering layer was further encompassed by the thin fibrous capsule which reflected from the dura mater or sinus wall. Both the outer and inner zones were characterized by numerous extracellular cisterns which appeared, electron-optically, to be empty or contain a little 'fuzzy' material. In the villi affected by subarachnoid hemorrhage, extracellular cisterns were distended by intact of disintegrating erythrocytes which appeared to be natural tracers of cerebrospinal fluid. The size of these cisterns measured approximately 10 microns in the maximum diameter. It is suggested that extracellular cisterns may contribute to the bulk outflow of cerebrospinal fluid.