Cerebral vasodilatory testing provides an important measure of both the hemodynamic significance of arterial occlusive disease and the adequacy of collateral pathways. This study measured the extracranial intravascular volume flow rate response to acetazolamide using phase-contrast magnetic resonance angiography. From 10 patients with unilateral carotid transient ischemic attacks (TIAs), a total of 18 extracranial carotid arteries (10 symptomatic, 8 asymptomatic, 2 occluded) and 19 extracranial vertebral arteries were studied. Patients were free of large-vessel intracranial stenoses, evident areas of ischemic infarction, evident areas of nonspecific white matter change, and hemodynamic or low-flow induction of TIA symptomatology. Asymptomatic carotid volume flow rates rose from 151 +/- 19 (standard error of mean) to 220 +/- 26 ml/min while symptomatic flow rates rose from 106 +/- 22 to 145 +/- 25 ml/min. Dominant vertebral volume flow rates rose from 128 +/- 23 to 160 +/- 22 ml/min while nondominant rates rose from 40 +/- 12 to 61 +/- 15 ml/min. Carotid volume flow rates were inversely proportional to percent stenosis for both baseline (r = 0.51, p < 0.02) and acetazolamide (r = 0.81, p < 0.001) data. Baseline-plus-acetazolamide volume flow rate techniques safely measure intravascular vasodilatory responses. Intracranial measurement techniques are being developed to further study cerebrovascular reserve using phase-contrast magnetic resonance angiography.