Ventilatory muscle function was examined at rest and during exercise on a cycle ergometer in 8 patients with moderate to severe chronic air-flow limitation (FEV1, 32 +/- 4% predicted) in air and in oxygen. The diaphragmatic electromyogram (EMG) was measured using an esophageal electrode. In addition, measurements of esophageal (Pes), gastric (Pga), and transdiaphragmatic (Pdi) pressures and abdominal wall movements were made. Patients exercised to exhaustion at a constant submaximal workload (80% of maximal power output) inspiring air or 40% O2 in random order on separate days. At end-exercise in air, tidal inspiratory Pes swings were 36 +/- 4% of static maximal inspiratory Pes, and inspiratory Pdi swings were 45 +/- 7% of the static maximal Pdi. Arterial oxygen saturation decreased from 91 +/- 2% at rest to 80 +/- 5% at end-exercise in air. During exercise in air, 5 patients demonstrated a persistent and greater than 20% fall in the ratio of high frequency (150 to 350 Hz) to low frequency (20 to 46 Hz) power (H/L) of the diaphragmatic EMG, indicating impending diaphragmatic fatigue, and 2 patients had paradoxical motion of the abdominal wall. Exercise time at the same constant work load increased from 3.0 +/- 0.6 min in air to 6.4 +/- 1.2 min in O2 (p less than 0.005). At the comparable time during exercise in O2 to end-exercise in air, minute ventilation was less by 13% (p less than 0.005), which was entirely attributable to a lower frequency of breathing. Mean inspiratory and expiratory flows and heart rate were all significantly lower.(ABSTRACT TRUNCATED AT 250 WORDS)