Controlled hypotension is not infrequently induced to diminish operative bleeding, and in neurosurgical procedures, for the additional purpose of decreasing brain tension. There is considerable question as to whether the advantages of the procedure may not be outweighed by the attendant hazards.* The present study of cerebral hemodynamics and metabolism during acute and relatively prolonged blood pressure reduction was undertaken in hopes that it might yield pertinent information concerning the above problem. Method The 21 subjects of the present study were hospitalized patients of various age groups, in most instances having hypertension. Scheinberg and Stead's modification3 of the Kety and Schmidt procedure4 for the determination of cerebral blood flow (CBF) was applied in this study. The gas mixtures utilized were those described by Kety.4 Blood oxygen was determined by the manometric technique of Van Slyke and Neill.5 Mean arterial blood pressure (MAP) was obtained directly from