The liability to fatigue of the respiratory center is a subject which needs to be studied. Davies, Haldane, and Priestley (1) were the first to investigate the manner in which breathing adapts itself to increased resistance, and the point at which the adaptation begins to fail. They showed that the normal response in man to respiratory resistance in both phases of respiration is slow and deep breathing. When the resistance is excessive respirations become progressively shallower and more frequent and the subjects then become cyanotic. Haldane and his coworkers believe that this is due to fatigue of the respiratory center. They believe anoxemia hastens greatly the onset of fatigue and the ease with which it is produced. They conclude that the mechanism involved in the immediate response is the HeringBreuer reflex, pointing out that as a result of resistance, the time required for inflation or deflation of the lungs to reach the point at which the Hering-Breuer stimulus becomes effective is prolonged, that CO~ accumulates in the meantime, and that the next respiration is deep and vigorous. The more or less sudden onset of rapid, shallow breathing Haldane interprets as evidence of fatigue of the respiratory center, with a resulting predominance of the peripheral stimuli over the central impulses normally governing breathing.A study of these effects in animals was undertaken by us with several points in mind. We hoped for additional information as to the nature and origin of rapid and shallow breathing, which we have previously considered in both clinical and experimental studies (2-5). It seemed highly desirable to learn something about the liability to fatigue of so vital a structure as the respiratory center. Transferring the problem