Homeostatic mechanisms to compensate for the anoxia of high altitude are presented by several systems of the body and may be either rapid or delayed. Increased respiration and increased cardiac output, which tend to minimize the decrease of the pressure at which oxygen is delivered to the tissues, are immediate responses to oxygen want. In contrast, increased oxygen capacity of the blood is produced only after several days of acclimatization to anoxia, as in life on mountains. A third compensatory ad-. justment which has been suggested, but not demonstrated, involves changes in the intracellular enzymes such as to produce more efficient utilization of oxygen at lowered pressure. Of these three mechanisms, response to the acute oxygen want of aviation must rely essentially on the rapid respiratory and circulatory adjustments to diminish tissue anoxia. The single physiological datum which most sensitively reflects the extent of these adjustments is the mean venous oxygen tension (MVpO2), which represents the average tension at which oxygen leaves the blood at the venous end of the capillaries. The MVpO2 has been measured directly by several observers (1, 2) on blood obtained by puncture of the right side of the heart or catheterization of the right auricle, but these methods are hardly suitable for widespread use. Indirect measurement, by equilibration of suitable gas mixtures with pulmonary arterial blood in the alveoli, has been reported by a number of observers (3 to 9) since first used by Plesch in 1909 (3). These investigations, which were concerned with measurement of cardiac output employing the Fick principle, have been reviewed in detail in Grollman's monograph (10). The methods used have been largely discarded during the past decade. The possibility of application to the problems of aviation medicine and the availability of a simpler method of measuring oxygen tension have led us to reconsider the technic of indirect measurement of the MVpO2. In seeking optimal conditions of equilibration and assuring the reliability of the method finally adopted, several thousand determinations of oxygen tension were made by a nearly instantaneous physical method with an accuracy of 0.5 mm. Hg. The data presented here are only a small part of those available to show the reproducibility of the results.