Acetazolamide (ACTZ) vasodilating properties are used for the assessment of cerebral vasodilatory reserve not only in cerebral pathology investigation, but also in clinical pharmacology studies. However, the kinetics of these vasodilating properties are not clearly established; moreover, the cerebral selectivity of ACTZ-induced vasodilation has not been demonstrated. To address these issues, we performed an ACTZ test in 9 healthy volunteers and measured noninvasively the effects exerted by the drug simultaneously on common carotid artery blood flow (CCABF), middle cerebral artery mean blood flow (BF) velocity (MV) (transcranial Doppler technique), and facial cutaneous BF (CutBF). The time of the peak increase in MV was variable, occurring between 20 and 40 min after ACTZ, and brief. In addition, simultaneous increases (with variable mean peak times) occurred in flows in the CCA (+11% at 20 min) and in its intracranial (+25% at 30 min) and extracranial (+134% at 50 min) branches. The first finding suggests that the cerebral effects of ACTZ should be measured repeatedly to detect in each individual subject the time of MV peak increase; the transcranial Doppler is particularly suitable for such an approach. The second finding suggests that, especially in patients, extracranial actions of ACTZ must be taken into account for estimation of the cerebral vasodilatory reserve.