Oxygen saturation in six subjects wearing a nasal cannula at altitudes of 14,000 ft (4267 m), 20,000 ft (6096 m), 22,500 ft, (6858 m) and 25,000 ft (7620 m) was measured using an ear oximeter (Hewlett-Packard 47201A). Saturation was measured during four activities: at rest breathing through the mouth, at rest breathing through the nose, performing biceps-curls, and talking. Oxygen flow rates of 1.5-2.5 LPM NTPD were used. There was no difference in the saturation levels whether the subject breathed through the nose or through the mouth, and cannula position in the nose was critical to good oxygen saturation. During talking and while exercising, subjects' saturations varied more than during rest conditions, sometimes falling to unacceptable levels. Based upon the lack of hypoxia symptoms and the high levels of oxygen saturation up to 20,000 ft (6096 m), it is concluded that at flow rates of 1.5-2.0 LPM NTPD the nasal cannula can be safely used to maintain adequate oxygenation in healthy individuals in hypobaric chamber operations, aircraft flight, and other operations at altitude. Above 20,000 ft (6096 m), the cannula does not provide sufficient oxygenation for persons to perform these physical activities.