Cerebral blood flow (CBF) falls as hematocrit (Hct) rises. Investigators have differed on the relative importance of the increases in arterial O2 content (CaO2) and red blood cell concentration in mediating the fall. Our experimental protocol attempted to determine the independent effects of these two variables. In 13 unanesthetized lambs (less than 7 days old) we measured arterial and sagittal sinus blood gases, and O2 contents, and CBF (microsphere technique) at oxyhemoglobin Hcts of approximately 20 and 40% and after an isovolemic exchange transfusion with a mixture of normal and pure methemoglobin (MHb) containing red cells. Following MHb exchange, Hct rose (19.7 +/- 0.3 vs. 38.2 +/- 0.4%, mean +/- SEM) with little change in CaO2 (9.3 +/- 0.2 vs. 10.0 +/- 0.3 vol%). Arterial PCO2, pH, mean arterial blood pressure, and cerebral O2 consumption (CMRO2) did not change. However, CBF fell (153 +/- 11 vs. 110 +/- 7 ml . 100 g-1 . min-1). CBF declined further when CaO2 rose (17.3 +/- 0.5 vol%) at the higher oxyhemoglobin Hct (36.9 +/- 0.8%). We calculated that the increase in red cell concentration accounted for 56% of the decrease in CBF that ordinarily occurs as Hct rises from 20 to 40%. The effect of red cell concentration on CBF varied among individual animals. It correlated closely (r = -0.77) with the initial cerebral fractional O2 extraction [E = CMRO2/(CBF X CaO2)]. Animals with the most luxuriant O2 supply (CBF X CaO2) relative to demand (CMRO2) had the greatest decrements in CBF as red blood cell concentration rose.