The objectives of this investigation are: a) to continue to expand our knowledge of the physiology of the control of the resistance function of the cerebral vasculature; b) to explore new approaches to the therapeutic control of cerebral vessels (1) by studying the effects of humors, neurohumors and drugs reaching the cerebral vessels via the "blood-cerebrovascular barrier" or via the "cerebrospinal cerebral vessels barrier" and (2) by studying possible interactions among humors, neurohumors and drugs on their effects on the cerebral vasculature; and c) to expand to primates our studies on the nervous control of the cerebral circulation. The methods used to study the control of the resistance vessels will consist of measurements of blood flows and pressures in anesthetized animals and of quantitation of vasomotor tone and vascular reactivity. Intra-arterial and ventriculo-cisterna perfusions will be used to study the effects of changing the blood and the cerebral interstitial fluid concentration of humoral, neurohumoral and drug agents on the cerebral circulation. Stereotoxic techniques will be used for the placement of electrodes to stimulate central nervous system centers.