We investigated whether blood flow determined by a flow probe situated on one common carotid artery provided an accurate estimation of unilateral cerebral blood flow (CBF) in piglets. In eight anesthetized, mechanically ventilated piglets, blood flow determined by an ultrasonic flow probe placed on the right common carotid artery was correlated with CBF determined by microspheres under two experimental conditions: 1) before ligation of the right external carotid artery with both the right external and internal carotid circulations intact [common carotid artery blood flow (CCABF) condition], and 2) after ligation of the right external carotid artery (ipsilateral to the flow probe) with all residual right-sided carotid artery blood flow directed through the right internal carotid artery [internal carotid artery blood flow (ICABF) condition]. The left carotid artery was not manipulated in any way in either protocol. Independent correlations of unilateral CCABF and ICABF with microsphere-determined unilateral CBF were highly significant over a 5-fold range of CBF induced by hypercarbia or hypoxia (r = 0.94 and 0.92, respectively; both p < 0.001). The slope of the correlation of unilateral CCABF versus unilateral CBF was 1.68 +/- 0.19 (SEM), suggesting that CCABF overestimated CBF by 68%. The slope of the correlation of unilateral ICABF versus unilateral CBF did not differ significantly from unity (1.06 +/- 0.15), and the y intercept did not differ significantly from zero [-1.3 +/- 5.2 (SEM) mL]. Consequently, unilateral ICABF determined by flow probe accurately reflected unilateral CBF determined by microspheres under these conditions. Flow probe assessments of CCABF and ICABF in piglets may provide information about dynamic aspects of vascular control in the cerebral circulation that has heretofore been unavailable.