This is a proposal to study the acute phase of regional cerebral ischemia in cases of stroke in evolution, recently completed stroke, vasospasm in subarachnoid hemorrhage, and vasospasm following craniotomy for ligation of aneurysm. The nucleus of this study is the repetitive measurement of regional cerebral blood flow by Xenon 133 inhalation sufficiently frequently to detect evidence of developing or failing collateral circulation, propagation or lysis of thrombus, as manifested by regional blood flow changes. These measurements will be correlated with repetitively measured or continuously monitored clinical state, blood gases, blood pressure, and EEG.