Arachnoid granulations and villi from 23 brains from subjects aged 9 to 84 years were examined post mortem by serial sections with the light microscope and as whole or fractured preparations in the scanning electron microscope. The object of the study was to investigate the pathways within the arachnoid granulations by which cerebrospinal fluid (CSF) drains from the subarachnoid space to the sinus endothelium. At the base of each granulation, a thin neck of arachnoid projects through an aperture in the dural lining of the sinus and expands to form a core of collagenous trabeculae and interwoven channels. An apical cap of arachnoid cells, about 150 microns thick, surmounts the collagenous core, and channels extend through the cap to reach subendothelial regions of the granulation. Channels within the granulation are lined by compacted collagen and may contain macrophages. Following recent subarachnoid hemorrhage, erythrocytes are found in the channels, suggesting that the channels are in continuity with the subarachnoid space and are CSF drainage pathways. The cap region of the granulation is only attached to the endothelium over an area 300 microns in diameter; the rest of the granulation core is separated from the endothelium by a subdural space and a fibrous dural cupola. Scanning electron microscopy reveals an intact endothelial surface to the granulations with small perforating venous channels present on the apex of some granulations. The differences between human arachnoid granulations and arachnoid villi in animals are discussed, together with preliminary observations regarding the transition of villi into granulations in man.