SUMMARY The relationship between lung volume and regional distribution of ventilation measured by the single breath oxygen test has been found to be similar in the upright and supine positions. When subjects breathe at progressively smaller lung volumes there is an increase in the slope of the alveolar plateau indicative of a redistribution of ventilation to the uppermost parts of the lung. On assuming the supine position the expiratory reserve volume falls and because of this change in volume there is a similar redistribution of inspired gas away from the well perfused base. The mechanics of these changes are discussed.