We evaluated the effects of acute hypovolemic hypotension followed by rapid, corrective blood infusion upon sequential regional cerebral blood flows (rCBF) in nine anesthetized 24-48-h-old beagle pups using the radioactive microsphere technique. After 5 min of hemorrhagic hypotension, during which time systemic arterial blood pressures were reduced 35-45%, cerebral blood flow to all regions increased by 31-42% over steady-state values. After blood reinfusion, rCBF increased by 39-88% over control values. Highest blood flows during hypovolemia were in the medulla and pons; slightly lower values were in the cerebellum, midbrain, cortex, and thalamus; and the lowest flows were in the white matter. These flows paralleled the hierarchy of flows to these regions in the steady state. Increases in blood flow during hemorrhagic hypotension were statistically significant when compared with the control values in the medulla (P less than 0.01), pons (P less than 0.01), midbrain (P less than 0.01), cerebellum (P less than 0.01), and cortex (P less than 0.01). One minute after reinfusion of the withdrawn blood volume, blood flows further increased by substantial margins in the medulla and pons; by moderate degree in the thalamus, midbrain, and cerebellum; and mildly in the cortex and white matter. Reinfusion state flow increases were significant when compared with control values in the medulla (P less than 0.01), pons (P less than 0.01), midbrain (P less than 0.01), cerebellum (P less than 0.01), cortex (P less than 0.05), and white matter (P less than 0.01). After flow values were corrected to compensate for variations in PCO2, the increases remained significant during both hypovolemia and reperfusion for medullary, thalamic, cortical, and white matter flows. Three of the nine puppies had macroscopic intraventricular hemorrhages at autopsy.