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To evaluate whether a spontaneous increase in cerebral blood flow (CBF) could be observed in subjects with severe hypertension and to study the effect of a calcium antagonist, felodipine, on blood pressure and CBF after acute and chronic administration.Patients with severe hypertension were recruited at the emergency ward. Patients with previous treatment with calcium antagonists, women of child-bearing potential, severe uraemia, nephrotic syndrome, heart failure, manifest cerebrovascular lesions and pathological liver function tests were excluded.CBF was measured by single-photon emission computed tomography after intravenous administration of xenon-133 before (CBF1) and after intravenous infusion of felodipine, 0.01 mg/min during 40-60 min (CBF2) in 12 patients aged 25-67 years with no antihypertensive treatment except for P-blockers in four patients and P-blockers plus a diuretic in one patient. CBF was repeated after 3 weeks of oral therapy with felodipine, 5-10 mg twice a day with the addition of p-blockers in 10/12 patients (CBF3).During the felodipine infusion blood pressure decreased. There were no neurological symptoms or signs before or during the felodipine administration. CBF1 was within normal limits with no significant differences between previously treated and untreated patients. There was a non-significant tendency to increase in global CBF after felodipine administration, associated with a significant reduction in the physiological side differences in blood flow.In spite of the initially very high blood pressure, no general or focal hyperaemia was observed, and thus no evidence for a 'breakthrough' of the cerebral autoregulation. Felodipine gives a smooth blood pressure reduction with a maintained CBF.