To elicit evidence concerning neurogenic control, regional cerebral blood flow determined by measurement of cortical temperature was examined in monkeys. Following three hours of temporary occlusion of the MCA, pressure autoregulation was preserved in all control animals. Presumptive partial chemical sympathectomy, produced by the administration of either L-alphamethyl- tyrosine or 3-alpha-dimethyl-tyrosine methyl ester HCI, was associated with loss of pressure autoregulation following 1.5 hours of occlusion of the MCA on only the side of the occlusion. Failure of pressure autoregulation in the treated animals implies that sympathetic control was a partial requirement of proper postischemic pressure autoregulation.