Ultrastructure of the air-blood barrier and surface surfactant activity were studied at different time periods of nonspecific inflammation of the lungs in guinea pigs. The animals were sacrificed 3 days, 2 weeks and 1, 2 and 4 months after beginning of the experiment. It has been demonstrated that in early periods of lung inflammation there was edema of all components of the air-blood barrier. Subsequent development of inflammation is accompanied by surface activity decrease associated with dystrophic changes in the epithelial cells of alveoli. At the same time there are compensatory changes in the lungs, directed to eliminate deficiency of surfactant.