Desynchronizatio n of the electrocorticogr am due to activation of cortical neurons is maintained by specific hemodynamic and biochemical changes which provide the energy for nervous activity currently taking place (1, 2, 22, 25, 28). This view is based on investigations of neurons of the sensomotor cortex (4, 7, 26, 27) in which desynchronization of the ECoG arising during transition from "slow" to "fast" sleep, (in response to a) stimulation of the posterior hypothalamus, b) brainstem reticular formation, c) action of photic stimulation, or d) during defensive or orienting reflexes) was accompanied by an increase in the cerebral blood flow, an increase in the O z tension and assimilation of oxygen and glucose, and by an increase in the content of succinate dehydrogenase and proteins containing sulfhydryl groups. However, the problem of the sources of energy for mutual activation or inhibition of cortical and subcortical centers, a problem posed by Parlor in 1930, still remains "one of the most intriguing tasks in modern neurophysiology" (2). The investigation described below was designed to study the cortical circulation during stimulation of nuclei of the posterior hypothalamus and mesencephalic reticular formation, and to determine the changes taking place in their hemodynamic influences during stimulation of the premotor cortex, hypoxia, hypercapnia, blocking of the carotid sinuses, and vagotomy. METHOD In acute experiments on cats anesthetized with urethane and chloralose (400 and 60 mg/kg respectively) electrodes were inserted into the premotor cortex, the medial mamillary nucleus of the posterior hypothalamus, and the reticular nucleus of the midbrain in accordance with the stereotaxic coordinates of Fifkova and MarSala (21). Steel electrodes, 0.15 mm in diameter, in a glass sheath were fixed with acrylic cement. The reference electrode (a silver disc) was placed on the animal's tongue. Square pulses (0.2 msee; 4v for the hypothalamus and reticular formation and 10-12v forthe cortex) were applied from a Multistim stimulator. The frequency of stimulation was 60 Hz for the cortex, 100 Hz for the hypothalamus, and 200 Hz for the reticular formation. The time between stimuli was not less than 10-15 rain. Localization of the electrodes was verified by histological study of serial brain sections. The blood supply in the cortical vessels was determined from the venous outflow from the sagittal sinus which drains blood from the rostral parts of the cat's brain, including the superomedial zones of the hemispheres (except the olfactory bulbs), by means of a bubble flowmeter of our own design (11). Because of the high accuracy of measuremerit and low internal resistance of the flowmeter, the method of recording the cortical blood flow suggested in 1956 by Ingvar and Soderberg (24) could be modified slightly. The