OBJECTIVE The aim of our study was to evaluate by transcranial Doppler ultrasonography the dynamics of blood flow velocity changes in the middle cerebral artery during and after hypocapnia-induced vasoconstriction in untreated essential hypertensive patients. METHODS Sixteen hypertensive patients (10 men and six women, 29-62 years of age) and 10 healthy control subjects (six men and four women, 30-62 years of age) were studied. Patients with mild-to-moderate essential hypertension (mean +/- SE blood pressure, 171/106 +/- 3/2 mm Hg) belonged to stage I or II of the World Health Organization classification. Mean blood flow velocity in the middle cerebral artery, arterial blood pressure, and end-tidal CO2 partial pressure were recorded at baseline, during 2-minute hyperventilation, and every 30 seconds up to 5 minutes after hyperventilation. RESULTS End-tidal CO2 partial pressure values overlapped in the two groups throughout the study. Baseline values of mean blood flow velocity in hypertensive patients were similar to those in normotensive subjects (mean +/- SE values, 64.7 +/- 3.9 cm/sec versus 58.6 +/- 3.7 cm/sec). A similar fall in mean blood flow velocity was observed in hypertensive patients and normotensive subjects (43.2 +/- 2.8% versus 46.7 +/- 3.6%). Mean blood flow velocity reverted to baseline more quickly in hypertensive patients: 1.5 minutes after hyperventilation, mean blood flow velocity was 60.7 +/- 3.1% and 84.9 +/- 1.8% of control in normotensive subjects and hypertensive patients, respectively. No changes in arterial blood pressure were observed in either group throughout the study. CONCLUSIONS This study demonstrates that the recovery of blood flow velocity in the middle cerebral artery after hyperventilation is faster in hypertensive patients than in normal subjects, thus providing further evidence that chronic hypertension is associated with changes in the dynamics of cerebral blood vessel reactivity.