To assess the effects of extreme exercise on lung function we measured maximal expiratory flow volume (MEFV) curves in fifteen runners (mean age, 35.3 years; range, 26-46) before and after an 80.6 to 100 km (50-62.2 mile) road race. Mean running time for 80.6 km was 7 hr, 42 min. Post-race testing showed significant decreases of 12.4 percent in forced vital capacity (FVC), 9.5 percent in forced expiratory volume in one second (FEV1) 13.7 percent in peak expiratory flow (PF), and 28.4 percent in flow at 50 percent of FVC (MEF50). By 2.5 hours after the race lung function had improved. The reduction in flow rates after ultramarathon running may be due to airway obstruction. In contrast, the decrease in FVC with gradual recovery of lung function after rest and nourishment suggests the development of respiratory muscle fatigue.