Left atrial catheters are used to measure left heart filling pressure in patients after open-heart surgery. It was observed that in some patients blood gases obtained from the left atrial catheters had a markedly higher PO2 as compared to PaO2 in the presence of severe hypoxemia. Twenty-five patients were studied consecutively; pulmonary venous admixture calculated from arterial blood was higher in 19 patients and lower in 5 as compared with that calculated from blood withdrawn from the left atrial catheter. These differences in venous admixture are due to regional changes in the lungs. This observation can be utilized in concentrating respiratory therapy to the regions of the lungs involved with significant therapeutic benefit to the patient.