The ability of a pulsed oxygen delivery system (Puritan-Bennett Companion Oxygen Saver (COS-5) to track respiratory rate during exercise and the oxygenation achieved during the exercise while oxygen was being delivered by this system was compared to that attained while oxygen was delivered continuously in six patients with chronic obstructive pulmonary disease (COPD) and six patients with idiopathic pulmonary fibrosis (IPF). The COS-5 appeared to respond appropriately at respiratory rates between 15 and 45, even when there were minimal pressure changes at the nose. There was an excellent correlation in PaO2 at equivalent flow settings during the exercise in the patients with COPD and IPF. There were six instances (in four patients) of the 31 comparisons in which the PaO2 differed by more than 5 mm Hg. The PaO2 was higher with continuous oxygen delivery on two occasions in a patient with COPD who utilized pursed-lip breathing during the exercise. The PaO2 was higher with COS-5 delivery on two occasions in a patient with IPF who was breathing at the highest respiratory rates (44 and 45/min) during the exercise.