Oxygen tensions of 50 simultaneously withdrawn blood samples from the right atrial orifice of a triple-lumen pulmonary artery catheter and from the pulmonary artery lumen were compared. Mixed venous oxygen tensions ranged from 21 mmHg to 44.9 mmHg. The correlation coefficient was 0.96 (t = 2.09; P = 0.01). The benefits and drawbacks of intermittent right atrial oxygen tension monitoring are discussed.