Regional distribution of ventilation and perfusion in the lung has been studied using Xe133 in normal subjects. In 31 subjects seated upright at rest the previous findings of a gradient of ventilation and perfusion distribution from apex to base of the lung, have been confirmed. The results agree well with those obtained using C15O2. In seven normal subjects lying supine, the V/Q distribution from apex to base of the lung is much more uniform, though a perfusion gradient can be shown to exist from front to back. Five normal subjects exercising on a bicycle ergometer in the upright posture were found to have a proportionately much greater blood flow through the upper zone of the lung than when in the same position at rest. One study during induced syncope on standing indicated that this state is accompanied by a progressive decrease in perfusion to the upper zones of the lung. No change in distribution occurred breathing 100% oxygen. xenon 133; posture; exercise; syncope; oxygen Submitted on August 30, 1963