1. The breathing pattern in normal subjects during exercise was compared with that in patients with obstructive and restrictive lung defects. 2. In most normal women and patients with obstructive or restrictive lung disease, as the frequency of breathing increased both inspiratory and expiratory duration fell. However, in most normal men (74%) inspiratory duration did not fall as ventilation increased. 3. Women breathed faster than men, and both obstructed and restricted patients breathed faster than normal subjects. 4. The airflow patterns in normal men and women were similar, but most patients with restrictive or obstructive lung disease showed an approximately exponential fall in flow during expiration.