The cervical sympathetic chain was stimulated electrically at 6 or 3 Hz on one side in anesthetized cats. Acute arterial hypertension was induced by ligation of the aorta. Evans blue was given as tracer for protein leakage. The regional blood flow in the brain was determined by using labelled microspheres. At high blood pressures there was a multifocal breakdown of the blood-brain barrier. The regions with breakdown had 10-20 times the normal flow rates. With a maintained hypertension regions which were overperfused at 5 min were still overperfused at 10 min, but there was little addition of new overperfused areas. Normalization of the pressure resulted in almost twice the normal flow rates in previously overperfused regions. The breakdown of the blood-brain barrier was restricted to the non-stimulated side, or more marked on that side. The protective effect of the sympathetic stimulation lasted more than 10 min. The results indicate that acute arterial hypertension tends to cause forced and long-lasting vasodilation in some areas in the brain but regions which are resistant to the acute rise have an increase in the vascular tone. Sympathetic activity helps in developing this tone. Normalization of the blood pressure results in partial recovery of the vascular tone in previously overperfused regions and normalization in other areas.