The effect of oxygen breathing on pulmonary compliance was studied in paralyzed, artificially ventilated dogs and in two normal subjects. 1) In the dogs and the human subjects the course of compliance during oxygen breathing for periods of 1frac12–2 hr following hyperinflation was compared with that during air breathing. 2) In dogs compliance after apnea on oxygen (following hyperinflation) for frac12–7 min was compared with that after apnea on air. Intrapulmonary pressure during apnea was atmospheric. 3) In the human subjects the course of compliance was followed during air breathing for 50–70 min, preceded by hyperinflation and immediately followed by oxygen breathing for 80–100 min. In none of these three sets of studies did oxygen breathing affect the course or magnitude of pulmonary compliance. It is concluded that breathing pure oxygen at atmospheric pressure for several hours does not enhance the development of pulmonary atelectasis in normal subjects. Submitted on July 3, 1961