Experiments were carried out at sea level to investigate tolerance and work capacity of test subjects breathing 100% oxygen and hypoxic gas mixtures (HGM) containing 12.7-12.8% O2 at a positive pressure of 800 mm H2O. All test subjects tolerated 1 hour exposure to 100% O2 or HGM under positive pressure. However, HGM induced more distinct cardiopulmonary changes than O2. During the first minutes of exposure to positive pressure breathing the tracking performance deteriorated most. The number of errors was 24.5% with O2 and 21.4% with HGM more than in the pretest study. As the adaptation developed, the number of tracking errors decreased to reach the pretest level with O2 and to remain 6-10% higher than the pretest level with HGM.