The letter from Drs. Stefan, Leschin, and Gray raises the important question of the validity of measurements of the partial pressure of oxygen (PO2) in capillary blood. We concur that measurements of oxygen tension are essential in monitoring ambient oxygen levels, and we would agree that, in the absence of shock, values on capillary blood reflect the levels in arterial blood. Fortunately, data exist, at least in normal newborn infants breathing air, that permit evaluation of the usefulness of capillary samples.