Arachnoid villi from cynomolgus monkeys subjected to various states of cerebrospinal fluid (CSF) absorption have been examined with scanning (SEM) and transmission (TEM) electron microscopy. Pressures within the superior sagittal sinus and the subarachnoid space were rigidly controlled, both prior to and during perfusion fixation and, as such, we created and studied conditions of normal, absent and increased cerebrospinal fluid absorption. Under normal conditions, the most prominent feature of the CSF/blood interface was the presence of endothelial intracytoplasmic vacuoles. The presence of these vacuoles was suggested with SEM and readily confirmed with TEM. Occasionally, these vacuoles coalesced with both the CSF and sagittal sinus fronts, thereby creating transcellular channels as identified by TEM or surface pores as seen with SEM. Villi perfused during conditions of no CSF flow exhibited none of the previously described vacuoles, channels, or pores, whereas increased CSF flows were associated with increased numbers of these structures. The significance of these findings was discussed in relation to CSF absorption and to previously reported ultrastructural studies.