The effects on cerebral hemodynamics of venisection and a 4% albumin-saline infusion were studied in six patients with high hematocrit (mean, 51.5%). Cerebral blood flow (CBF) was measured using the xenon 133 intracarotid injection method. Blood gases were measured in arterial and jugular venous blood. Rapid two-stage hemodilution, which lowered mean hematocrit by 9 and 13%, resulted in CBF increases of 19 and 23%, respectively. Jugular venous partial pressure of oxygen and oxygen delivery capacity (CBF x arterial oxygen content) did not change significantly from baseline. The cerebral metabolic rate for oxygen increased slightly following stage 1 hemodilution but returned to baseline value following stage 2. The study lends no support to the concept that patients whose hematocrit is at the high end of the normal range have generalized cerebral hypoxia.