In specimens from the superficial temporal artery (STA) and middle cerebral artery (MCA), obtained during STA-MCA anastomosis, green fluorescent varicose fibers of sympathetic nerves were clearly visible with both formaldehyde-glutaraldehyde and sucrose-potassium phosphate-glyoxylic acid wet-histofluorescent techniques. These fibers were fairly thick, were densely packed and had a meshwork-like arrangement. Fluorescent terminals were seen both in the adventitia and in the outer muscular layer of the media in both STA and MCA specimens. They were more often observed in patients with prominent atherosclerosis in these vessels. The present study suggests the possible role of sympathetic nerve terminals in the development of vasospasm and occlusive lesions in cerebral vessels. It may also help to explain the marked constriction and transient occlusion following a STA-MCA bypass procedure.