A method for detn. of O2 consumption in an open circuit, using a Pauling O2 meter is descr. Nomograms are given for obtaining the O2 consumption /l. of inspired air from the data of partial pressure of O2 in the inspired and the expired air after removal of CO2. The data as found in the nomogram include correction for standard conditions. The method descr. has practically the same sensitivity as that of the conventional gas analysis method; but it is easier, faster and allows continuous detns. It has about the same sensitivity as the Benedict closed-circuit method. It seems more reliable when the breathing is irregular, since this does not affect the data obtained on an open circuit system.