The regional distribution of ventilation-perfusion ratio (VA/Q) in the lung was obtained from the continuous inhalation and infusion of 81mKr. With this short half-life isotope (13 s) serial measurements were made in six young normal subjects during spontaneous breathing, with minimal radiation exposure. At rest in the upright position the apical zone had a high VA/Q, but over the lower two thirds VA/Q distribution was relatively homogeneous. There was a slight increase towards the base. During exercise at 50 and 100 W the distribution became more uniform but there was more obvious than at rest. These distributions are consistent with a zone of reduced basal blood flow (zone IV) which becomes more marked on exercise. Apical blood flow was followed during the transition from rest to exercise and recovery. There was a large (40-150%) increase within 30 s of starting to pedal at 50 W, but little change thereafter.