This study reviews the effects of using oxygen saturation measurements during exercise and the effects of the method of exercise testing on the prescription of oxygen therapy. Using cutaneous oximetry (designated A and B models) and co-oximetry, 25 of 41 patients (model A), 30 of 39 patients (model B), and 28 of 42 patients (co-oximetry) had an oxygen saturation measurement greater than 85 percent at maximal incremental exercise despite an arterial oxygen tension less than 55 mm Hg. Also, in a group of patients who underwent incremental followed by constant work rate testing, 9 of 28 exhibited a fall in arterial oxygen tension to 55 mm Hg or below only during the constant work rate testing. Oxygen saturation measurements cannot reliably be used as a substitute for arterial oxygen tension measurements for the prescription of oxygen therapy. The type of exercise study performed may influence the outcome of such oxygen prescription.