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Cerebral blood flow was measured using 133 xenon in forty-five baboons lightly anesthetized with pentobarbital. Blood flow varied between 62 and 82 ml/100g/min in gray matter and between 15 and 21 ml/100g/min in white matter. Hypercapnia and hypoxia caused a rise in blood flow and a fall in vascular resistance. Blood flow was independent of mean arterial blood pressure over the range 60 to 130 mm Hg. Section of the cervical sympathetic nerve enhanced the vascular response to CO 2 . Stimulation of the sympathetic nerve caused a reduction in blood flow in proportion to the initial blood flow. Similarly, after sympathectomy, blood flow was uniformly higher than control over the range of Pa o 2 tested (35 to 450 mm Hg). After sympathectomy, blood flow was little different from control at low blood pressure but was higher within the physiological range and blood flow then fell steadily as pressure was reduced. When the carotid, vagus, and aortic nerves were cut in the neck, the blood flow response to hypoxia and hypercapnia was reduced, and when the aortic or vagus nerves were stimulated centrally, blood flow increased independently of Pa CO 2 . Section of the seventh cranial nerve caused small and variable changes in blood flow, but if the vagus nerves had previously been sectioned, stimulation of the seventh cranial nerve caused an increase in blood flow in some of the tests. These results indicate that cerebral blood vessels are under reflex control. Possible receptors and pathways involved are discussed.

Cerebral blood flow was measured with xenon 133 in 21 fetal sheep and 14 newborn lambs under 48 hours old. When the ewe breathed air, the average gray matter blood flow in the fetuses was 70.6 ml/100 g/min, the average white matter blood flow 15.4 ml/100 g/min and average gray matter oxygen consumption 2.06 ml/100 g/min. The corresponding values in the newborn lambs were 87.2, 17.8, and 2.98 ml/100 g/min. A reduction of fetal Pao 2 by 7 to 11 mm Hg at a constant Paco 2 or an increase of Paco 2 at constant Pao 2 caused a rise in blood flow and arterial pressure and a fall in vascular resistance. Intravenous catecholamines caused variable changes in blood flow but no significant change in gray matter oxygen consumption. Occlusion of the umbilical cord caused an average increase in gray matter blood flow of 50% of control, in white matter blood flow of 30%, and in gray matter oxygen consumption of 90%. By the end of the first hour, these values had fallen and were then similar to those seen in the newborn lamb. These changes were not seen in fetuses in which both vagi had been cut nor in two fetuses with a failing circulation.