We studied the effects of exercise on lung mechanics in young and older individuals. A constant pressure body plethysmograph was adapted to contain the pedals of a cycle ergometer, and the problems of vibration and temperature control were overcome. Two groups of healthy non smoking males, with no history of lung disease took part: 6 young subjects, aged 23-29, mean 25 yr were compared to 6 older subjects aged 48-65, mean 54 yr. Measurements of pulmonary mechanics were made in steady states of rest and during exercise at several power outputs between 200-800 kpm/min. We found no systematic difference between the 2 groups in the responses to exercise. In all subjects except one the maximum expiratory flow:volume curves and the quasi static pressure:volume curves obtained in exercise were superimposable on the curve obtained at rest, indicating that the mechanical properties of the lungs did not change on exercise. The exception was a young subject who consistently showed, with increasing exercise, a progressive increase in flow at given lung volume, without any change in elastic recoil. We concluded that in this subject upstream resistance fell with exercise.