1. The incidence of entrainment of breathing frequency by the rhythm of exercise was detected by a cross-correlation of the two frequencies. 2. During moderate, steady-state exercise on a bicycle ergometer at 50 rev/min, eight of fifteen volunteers (53%) showed entrainment when pedalling speed was kept constant with a metronome, and three of fifteen volunteers (20%) showed entrainment when pedalling speed was kept constant with a speedometer. 3. At 70 rev/min, in a second group of fifteen volunteers, the results were nine of fifteen (60%) and five of fifteen (33%) respectively. 4. During moderate, steady state exercise on a treadmill, in a third group of fifteen volunteers, eight of 15 volunteers (53%) showed entrainment while walking, and twelve of fifteen volunteers (80%) showed entrainment while running. 5. It is concluded that the rhythm of exercise is likely to affect the rhythm of breathing and that this controlling factor must be considered during studies of breathing pattern in exercise.