Patient with severe chronic obstructive pulmonary disease (COPD) can develop respiratory muscle weakness and fatigue. Arterial hypoxemia can predispose to this condition. To assess whether O2 administration improved respiratory muscle function and respiratory pattern in COPD 11 patients with chronic hypoxemia were examined. Each patient was first submitted to respiratory function studies, including lung volumes, in normal and recumbent position, respiratory frequency, Ti/Te, Ti/Tot, maximal inspiratory and expiratory pressures (Pimax, Pemax) and arterial gas analysis breathing room air. All these tests were repeated during oxygen Administration through a nasal cannula. The arterial PO2 improved from 50 +/- 7 mmHg to 81 +/- 28 mmHg but the breathing pattern, Pimax and Pemax as well as lung volumes remained unchanged. It is concluded that oxygen, in short term administration, doesn't influence respiratory pattern and muscle function.