The effects of various levels of blood COHb and of various simulated altitudes on the physiologic response to exercise were observed in a group of 10 normal men. The COHb conditions were: 0, 6, and 13% A COHb at simulated altitudes of 7,000, 10,000 and 15,500 ft. These altitudes were simulated by having the subjects breathe appropriate mixtures of O2 and N2 so as to produce the desired arterial O2 saturations. The physiologic response was examined by measurement of pulse rate, respiratory rate, and min. vol. of respiration. These functions were measured each min. during a 10 min. rest period, a 5 min. period of mild exercise, and a 10 min. exercise recovery period. Of these measures, the mean exercise pulse rate and the mean pulse rate during the first 15 min. following exercise exhibited the closest correlation with the exptl. variables, i.e., blood COHb and ambient O2 tension. On the basis of the changes in pulse rate, it was possible to derive a quantitative estimate of the effect of blood COHb on hypoxia tolerance. The increment in response/1% increase in blood COHb was equal to that obtained by raising a normal group 335 ft. in pressure altitude. This value was detd. for the altitude range of 7,000 to 10,000 ft. and for increases in blood COHb of at least 13%. The pulse rate response to exercise was recommended as a useful measure of hypoxia tolerance, and a standardized test based on it was outlined.