Fixation of the rat lung by perfusion through the pulmonary artery prevents the flushing of the macrophages into the lumen of the alveoli and maintains their natural distribution in the hypophase of the alveolar extracellular lining, beneath the film of surfactant. Surfactant synthesis is intensified in the large, alveolocytes of the remaining lung 5–7 days after left-sided pneumonectomy, the quantily of tubular myelin in the hypophase of the hypertrophied alveoli is increased, and the surface tension of the lung washings falls. The number of alveolar macrophages is more than doubled in this period. The alveolar macrophages utilize the “excess” of surfactant (tubular myelin) in the hypertrophied lungs and so participate in the regulation of the surface tension of the alveoli.