The progression of changes in cerebral blood flow (CBF) and neurological status were measured in 12 patients in whom profound hypotension (mean arterial blood pressure (MABP): 30 to 40 mm Hg) was used during intracranial aneurysm surgery. Nine patients (Group I) showed autoregulation of CBF to an MABP of 40 to 50 mm Hg during surgery. None of these patients had arterial spasm preoperatively. Postoperatively, mild flow disturbances were noted at the site of retraction. Three Group I patients developed arterial spasm postoperatively, but there was no associated neurological deterioration. The remaining three patients (Group II) had impaired autoregulation during surgery, and CBF decreased by 35% to 65% at an MABP of 50 mm Hg. Two of these patients had angiography immediately before surgery, and both showed moderate to severe arterial spasm. Relatively severe flow disturbances were noted postoperatively at the site of retraction, and two patients developed ischemic deficits of late onset. Brain retractor pressure and the degree and duration of hypotension were equivalent in the two patient groups. There was no correlation between intraoperative reductions in CBF (to as low as 20 ml/100 gm/min in the unretracted hemisphere) and immediate postoperative neurological deficits. The use of halothane and mannitol and the relatively short duration of the flow reductions were suggested as factors contributing to the protection from ischemia that was observed. Arterial spasm was found to produce hemodynamic instability and reduced CBF, although neurological status was unaffected in the majority of patients. Patients with impaired autoregulation during surgery were at increased risk of delayed ischemic complications postoperatively, and showed characteristic flow disturbances at all three stages of their clinical course.