This article, authored by K. Furusawa and communicated by Prof. A. V. Hill, investigates the oxygen intake during exercise while breathing oxygen-rich mixtures. The study builds on previous research by Hill, Long, and Lupton, who found that the maximal oxygen intake increased by 50% when breathing oxygen mixtures compared to air. The authors discuss potential errors in the method, particularly the contamination of expired gases with air, which can significantly affect the results. They emphasize the importance of ensuring airtightness in the equipment and carefully washing all components to avoid such contamination. Another potential issue, the change in nitrogen equilibrium, is deemed negligible compared to the risk of air leakage. The experiments, conducted with meticulous attention to detail, confirm the significant increase in oxygen intake when breathing oxygen-rich mixtures, attributed to an increased circulation rate. The highest recorded oxygen intake per minute per kilogram of body weight is 87.3 c.c. for subject K.F., a 50 kg individual, under severe and rapid effort while breathing a high percentage of oxygen. The findings support the conclusion that the increased oxygen intake is due to an enhanced circulation rate, likely involving the heart muscle's ability to adjust its output based on the degree of arterial blood unsaturation.This article, authored by K. Furusawa and communicated by Prof. A. V. Hill, investigates the oxygen intake during exercise while breathing oxygen-rich mixtures. The study builds on previous research by Hill, Long, and Lupton, who found that the maximal oxygen intake increased by 50% when breathing oxygen mixtures compared to air. The authors discuss potential errors in the method, particularly the contamination of expired gases with air, which can significantly affect the results. They emphasize the importance of ensuring airtightness in the equipment and carefully washing all components to avoid such contamination. Another potential issue, the change in nitrogen equilibrium, is deemed negligible compared to the risk of air leakage. The experiments, conducted with meticulous attention to detail, confirm the significant increase in oxygen intake when breathing oxygen-rich mixtures, attributed to an increased circulation rate. The highest recorded oxygen intake per minute per kilogram of body weight is 87.3 c.c. for subject K.F., a 50 kg individual, under severe and rapid effort while breathing a high percentage of oxygen. The findings support the conclusion that the increased oxygen intake is due to an enhanced circulation rate, likely involving the heart muscle's ability to adjust its output based on the degree of arterial blood unsaturation.