The reliability of tonometered venous and arterial blood for calibration was demonstrated. The readings of O2 and CO2 tensions in samples of venous and arterial blood correlated well with the readings obtained by calibrating the electrodes with humidified gases. The different equilibration times used for tonometry of gases in blood did not result in any significant differences; therefore, 10 min. in the tonometer for equilibration of the venous blood with gases is sufficient. Calibration of a blood gas apparatus with gas is satisfactory if the gas is properly humidified. Therefore, in the authors' laboratory the machines are calibrated with gases equilibrated in blood once a week; the rest of the time the calibrations are done with humidified gases only.