It was recently reported B that normally many cells in the mesothelial lining of the arachnoid villi are characterised by the presence of giant vacuoles of several micrometres diameter. All vacuoles were bound by a unit membrane and were essentially electron-optically empty. Serial section analysis by transmission electron microscopy (TEM) revealed that the vacuoles were in fact invaginations from the basal aspect of the cell surface and thus, in life, they must be in direct communication with the cerebrospinal fluid (CSF) in the subarachnoid space. A small proportion of vacuoles showed in addition openings on the apical aspect of the cell surface thereby constituting vacuolar transcellular channels. It was postulated that the vacuoles are responsible for the formation of a dynamic system of transcellular pores which allow the bulk outflow of CSF down a pressure gradient, and that this vacuolation cycle, in providing the requisite number of pores at any given time across the mesothelial barrier, is probably a controlling factor in the bulk outflow of CSF and in the maintenance of its fluid pressure within the subalachnoid space 6. In order to trace the exit pathway of CSF, we have investigated in cynomolgus monkeys the fate of an electron-dense tracer of colloidal dimensions frequently used by electron microscopists to detect channels and pathways through complex epithelial and endothelial layers. Following a satisfactory level of Nembutal anaesthesia, 1 ml of CSF was slowly withdrawn and simultaneously the same volume of colloidally suspended thorium dioxide (Thorotrast--particle size approx. 10 nm) was injected through a second needle inserted into the cisterna magna. After about 20 min the brain tissue was fixed with glutaraldehyde (3~ solution in 0.1 M cacodylate buffer at pH 7.4) using an intravital sustained pressure perfusion technique 1, 5, s. Pieces of superior sagittal sinus and adjacent brain tissue were carefully excised and processed for TEM and scanning electron microscopy (SEM). A direct and relatively undistorted view of arachnoid villi was obtained by carefully excising the superior wall of the dural sinus and its lacunae and by processing the tissue for SEM using a critical point drying technique.