Endurance properties of the inspiratory and expiratory muscles were compared with those of the flexors and extensors of the elbow in healthy volunteers. During a series of sustained contractions separated by rest intervals of one minute there was a progressive decline in the force produced by both muscle groups acting at the elbow and by the expiratory muscles. By contrast, the ability of the inspiratory muscles to generate force recovered completely within the one minute intervals. The decline in force during the first sustained contraction was similar for the inspiratory muscles and those acting at the elbow. During series of repeated brief maximal contractions (duty cycle 50%) the inspiratory muscles fatigued less than the other muscle groups. This apparent resistance to fatigue shown by the inspiratory muscles may be due to many factors including the central organization of their motoneurones and to local neuromuscular properties. Evidence supporting a contribution from intrinsic muscle properties is provided by studies of isolated curarized muscles.