Systemic arterial hypertension was induced with a special set up for hemodynamic stroke and intravenous injection of angiotensin (AT) or noradrenaline (NA) in 12 cats anesthetized with chloralose one hour after unilateral sympathectomy. Arterial hypertension was induced after reflex activation of the sympathetic nervous system (SNS) induced by graded hemorrhagic hypotension. during the reduction of systemic arterial pressure (AP) to 50-60 mm. Hg followed by an abrupt rise up to 230-240 mm Hg, the permeability of the blood-brain barrier (BBB) was largely impaired in the hemisphere under sympathectomy. During the AP reduction to 70-80 mm Hg and intravenous injection of AT (NA) followed by an abrupt rise of AP to 230-240 mm Hg, the derangement of BBB permeability of the intact hemisphere was found for the first time. At the same time the BBB permeability of the hemisphere under sympathectomy was found to be relatively preserved. Thus, the SNS controls the BBB permeability by changing blood pressure in exchange vessels.