Dyspnea, cyanosis and other signs and symptoms are, of course, evidence of want of oxygen and are usually abated in a patient who is benefited by oxygen therapy. Clinical experience, however, has led us to believe that reduction of the pulse rate is by far the most reliable single guide by which to judge benefit, or failure of benefit, from the administration of oxygen-rich atmosphere. It is therefore desirable that when oxygen therapy is instituted, as high a concentration as is mechanically available should be first begun. Meanwhile a careful charting of the pulse rate should be made at least every fifteen minutes. With high concentrations, approaching 100 per cent, the pulse rate will probably be reduced during the first few hours. If such an effect is not secured, careful adjustment of the method of administration having been made to assure the operator that an extremely high percentage of oxygen