The respiratory inductive plethysmograph, a monitor of ventilation that does not require a connection from the airway, was validated against spirometry during moderate exercise using a bicycle ergometer and a treadmill. One hundred seventy-three of 200 breaths during bicycle exercise from 6 young adults and 211 of 242 breaths during treadmill walking were within +/- 20% of the tidal volumes measured by simultaneous spirometry. The respiratory inductive plethysmograph appears to be a useful non-invasive monitor of ventilation during exercise.