Transcranial Doppler sonography, using a 2 MHz pulsed Doppler system, is suitable for non-invasive continuous monitoring of middle cerebral artery blood flow velocity during pharmacological studies. Methodological problems, and factors affecting cerebral blood flow have to be discussed (vasomotor spontaneous changes, intracranial blood pressure, blood viscosity, heart rate, vascular risk factors, etc.). Arguing from a previous study in 28 subjects (11 healthy volunteers, 8 hypertensive patients, 9 patients with middle cerebral artery stenosis), showing the hypotensive action of Nifedipine, without significant decrease of cerebral blood flow in 83% of cases, we emphasize the advantages and limits of this new non-invasive method for cerebral blood flow monitoring.