In Nembutal-anesthetized rabbits, oxygen consumption of the entire animals was determined by means of a metabolism apparatus, and its variations with different degrees of breathing activities were studied. Breathing activity was varied either via the Hering-Breuer reflex, or by using different depths of anesthesia, or by changes in the dead space or resistance to air flow. Measurement of breathing activity was made by mechanical or electrical integration of the electromyograms of the diaphragm. Oxygen consumption has a linear relation with breathing activity. That relation can be modified by changes in heart rate, by asphyxia or hyperventilation, by the degree of activity of the sympatho-adrenal system or by too much Nembutal. Once these factors were controlled was found that oxygen consumption depends more on the integrated muscle activity than on either ventilation or breathing rate. It is emphasized that in some conditions the amount of oxygen used by the breathing processes can become an important fraction of the total basal metabolism.