The changes of oxygen saturation (SaO_2) and cardiac index (CI) of 23 soldiers who had stayed in a simulated high altitude of 4 000m and 5 000m for 72 hours were observed with a Model 102 BIT BIOX IIA earprobe oximetry and impedance cardiogram. It was found that both SaO2 and CI decreased because of hypoxia, and the decrease became more severe with the elevation of the altitude and the prolongation of staying. At the end of the 24th hour of staying, the 2 parameters reached the lowest. These facts suggest that hypoxia more or less depresses the cardiac function. Four hours after the subjects returned to sea level, SaO2 restored the normal value but CI remained significantly lower than the control valuet which implies that the effects of acute hypoxia still existed at that time.