To assess the gas exchange in a lung mixed venous oxygen content (CvO2) is one of the most important factors connecting cardiac output and oxygen uptake. Unlike the other fundamental factor arterial oxygen content (CaO2), CvO2 cannot be easily measured. Therefore, estimation of CvO2 is in high relevance to clinical medicine as well as to basic research. Oxygen saturation (SpO2) and heart rate (HR) were measured for 13 healthy students (4 males and 9 females ages 20 to 24) of Yamagata Prefectural University of Health Sciences, who joined a study tour in Colorado from Sep. 19 to 27, 2012. The means and standard deviations for their height and body weight were 162.0 ± 9.5 cm and 58.0 ± 7.8 kg. They had no cardio-respiratory diseases. SpO2 and HR were measured using a pulse oximeter (PULSOX-2, KONICA MINOLTA) with their informed consents. They put their fingers into the oximeter in a sitting position to measure SpO2 and HR. Averaged values of SpO2 for 13 students were 98 ± 0.95, 97 ± 1.4, 94 ± 1.9, 93 ± 1.7 and 87 ± 2.8 % at the Narita Airport (altitude 130 m), Denver City (1,609 m), Estes Park (2,499 m), Hidden Valley (2,816 m) and the Alpine Visitor Center (3,595 m), respectively. Averaged HR values simultaneously measured with SpO2 were 75 ±11, 78 ±5.8, 74 ± 7.4, 92 ±11 and 85 ±7.6 beats/min. These values except for those at the Narita Airport were measured within 6 hours on the fourth day for staying in Denver. Despite more than 10 % decrease in SpO2 within 6 hours, no student felt unwell at the height of 3,595 m. Based on the Fick principle, a correlation of SpO2 in % with 1/HR in min/beats was described with a regression line that SpO2 = 2650(1/HR) + 60.6 with a coefficient of determination 0.600. With the standard oxygen content combined to hemoglobin in arterial blood 20.1 vol% by assuming the normal hemoglobin concentrations for each student, CvO2 was estimated from the intercept of the regression line as 12.2 vol%1). The difference between the CaO2 values obtained from SpO2 and the estimated CvO2 (CaO2 – CvO2) at the Narita Airport was obtained as 7.8 vol%, which was consistent with that estimated by a rebreathing method at similar altitude as Narita2). The CaO2 – CvO2 value at 3,595 m was 5.2 vol%, which was higher than that reported by Wolfel et al.3) 4.0 vol% at Pikes Peak 4,300 m. This difference is probably due to the lower CaO2 at the higher altitude. These results suggest that CvO2 can be estimated from the regression line between SpO2 and 1/HR measured at different altitudes. At low altitudes SpO2 in healthy subjects is hardly less than 90 % even with severe exercise, and in this sense the application of our noninvasive method to estimate CvO2 is limited. However, our method can be used by measuring SpO2 and HR with inspired gases at various low O2 concentrations which simulate high altitudes.