Besides the central chemo-reflexes in breathing and next to the peripheral chemoreceptors, which both regulate the action of the lungs via the ratio systemic circulation, another regulating system must exist, localized in the lung itself. The author arrives at this conclusion as a consequence of a literature study completed with his own observations. He believes this regulating system to be situated in the acinus, defined as that part of the lung, distal to the terminal bronchiolus. In this area the ‘receptor substance’ must be located in the alveolar wall structures and in the post-alveolar capillaries. This control system should be capable of adapting the ventilation-perfusion ratio to a wide variety of circumstances. This system is thought to be guided by the partial pressures of oxygen and carbon dioxide in the alveolar gas, and by the resulting hydrogen ion concentration in the acinar tissues. Probably the muscle proteins as well as the endothelial cells of capillaries and venules are to be considered as the ‘receptor substance’. Consequences of this conception as to the functional fate of a transplanted lung are in