The effects of ATP on cerebral blood flow velocity and vascular responses to hyperventilation using TCD in the middle cerebral artery were studied in 15 patients. Anesthesia was maintained by infusion of 1% procaine (1ml.kg-1/min) and inhalation of 1% enflurane-40%N2O-O2. Mean blood flow velocity in left middle cerebral artery was measured and cerebral vascular reactivity to hyperventilytion was assessed before and during the administration of ATP.The cerebral reactivity was expressed as the percentage change in mean blood flow velocity per unit change in PETCO2. During the administration of 1%ATP,a 34% reduction in MAP with mean blood flow velocity decreased significantly from 61.5 to 53.1cm/s (P