Tests were made to determine the hypoxic effect of 9.5% O2 in N2 on human subjects. In response to the inhalation of this mixture during a period of 20 min., the pulse rate was significantly raised, i.e., by about 20% of the resting value. The oxyhemoglobin saturation fell sharply with the beginning of the exposure period. Variable (but apparently significant) changes in the respiratory rate and the ventilation volume occurred. It is concluded that the main compensation to the low O2 exposure was made by the changes in cardiac output incident to the increased heart rate, and that increased ventilatory efficiency constituted a somewhat less important factor in this regard. An examination of the variance indicates that in general the functions tested do not constitute reliable criteria for physiological test use.[long dash]R. E. Smith.