BACKGROUND Neurologic injury after cardiopulmonary bypass (CPB) is a frequent and devastating complication of cardiothoracic surgery. Disordered cerebral hemodynamics during CPB has been implicated as an important factor in the etiology of these injuries. Evidence of disordered cerebral hemodynamics includes reports of a progressive time-dependent decrease in cerebral blood flow (CBF) during stable full-flow CPB. Low-flow hypothermic CPB has become a preferred technique for the management of pediatric patients undergoing surgical repair of complex cardiac lesions. Because CBF is already substantially reduced with the onset of low-flow CPB, we determined if a similar progressive decline in CBF occurs during the low-flow state. METHODS After induction of general anesthesia in seven baboons, CPB was instituted. alpha-Stat management of arterial blood gases was used. Animals were cooled at a pump flow rate of 2.5 l.min-1.m-2 until tympanic membrane temperature decreased to 18 degrees C. CPB flow was then reduced to 0.5 l.min-1.m-2 and maintained constant for at least 77 min. Thereafter, CPB flow was increased to 2.5 l.min-1.m-2 and baboons rewarmed to normal temperature. CPB was discontinued after return of cardiac function. CBF was measured before, during and after CPB by washout of intraarterial xenon 133. RESULTS Low-flow CPB resulted in a decrease in CBF to about 50% of the prebypass rate and about 30% of the value measured during full-flow CPB. Sequential measurements of CBF at 30-min intervals during low-flow CPB showed no time-dependent change in cerebral perfusion. CONCLUSIONS Although systemic flow is reduced to 20% of full-flow during low-flow CPB, CBF reduced by half is disproportionately preserved relative to systemic flow. Furthermore, there is no time-dependent change in CBF under these low-flow conditions.