We determined the effects of exercise on active expiration and end-expiratory lung volume (EELV) during steady-state exercise in 13 healthy subjects. We also addressed the questions of what affects active expiration during exercise. Exercise effects on EELV were determined by a He-dilution technique and verified by changes in end-expiratory esophageal pressure. We also used abdominal pressure-volume loops to determine active expiration. EELV was reduced with increasing exercise intensity. EELV was reduced significantly during even mild steady-state exercise and during heavy exercise decreased an average of 0.71 +/- 0.3 liter. Dynamic lung compliance was reduced 30-50%; EELV remained greater than closing volume. Changing the resistance to airflow (via SF6-O2 or He-O2 breathing) during steady-state exercise changed the peak gastric and esophageal pressure generation during expiration but did not alter EELV; breathing through the mouthpiece produced similar effects during exercise. EELV was significantly reduced in the supine position. With supine exercise active expiration was not elicited, and EELV remained the same as in supine rest. With CO2-driven hyperpnea (7-70 l/min), EELV remained unchanged from resting levels, whereas during exercise, at similar minute ventilation (VE) values EELV was consistently decreased. At the same VE, treadmill running caused an increase in tonic gastric pressure and greater reductions in EELV than either walking or cycling. We conclude that both the exercise stimulus and the resultant hyperpnea stimulate active expiration and a reduced FRC. This new EELV is preserved in the face of moderate changes in mechanical time constants of the lung. This reduced EELV during exercise aids inspiration by optimizing diaphragmatic length and permitting elastic recoil of the chest wall.