THIS paper describes the result of simultaneous measurement of the blood flow in 16 regions of the brain using the xenon 133 (133Xe) injection method1-3in acute apoplexy. The study demonstrates the existence of hyperemia in some regions in certain cases of this disease. It further demonstrates that the local hyperemia correlated with the angiographic finding of early filling veins, a correlation also noted by Cronquist and Laroche.4Of perhaps greater importance for the understanding of the pathophysiology of apoplexy are our finding of a local loss of the normal regulation to variations of arterial blood pressure and arterial carbon dioxide tension. All these abnormalities are considered to be a consequence of localized tissue acidosis and to constitute an example of the "luxury perfusion syndrome" of brain tissue recently discussed by Lassen.5 Methods Regional cerebral blood flow was studied by the133Xe residue detection method