Since the discovery of the aortic and carotid chemoreceptors, a large number of investigations have been performed upon these structures (for bibliography, see (13)). However there is still disagreement upon two points of fundamental importance. The first of these is the question of the existence of tonic activity. Some investigators (11, 8, 9) maintain that chemoreceptor reflexes are of great importance in the control of respiration under all conditions (normal as well as abnormal) while others (4) believe that these structures function chiefly as an emergency mechanism, of extreme importance during anoxemia, asphyxia, acidosis, and marked hypercapnia, but relatively unimportant in the control of normal breathing. The second point has to do with direct stimulation of the medullary centers by anoxia. Although experiments upon anesthetized animals indicate that anoxia usually stimulates the medullary centers only reflexly through the chemoreceptors, it has been reported (12) that anoxemia may stimulate the medullary centers directly if the anesthesia is sufficiently light. Since anesthesia may either intensify or depress chemoreceptor activity depending upon the nature and concentration of the anesthetic3 we decided to investigate these problems on trained unanesthetized dogs.

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