The aim of the present study was to evaluate the effect of hypobaric hypoxia on the changes observed within lung parenchyma, particularly in the surfactant system-forming structures. The experiment used male Wistar rats, 180-220g b.w. The animals were kept in a hypobaric chamber for 3, 10 and 30 days. Pressure in the chamber (380mm Hg) corresponded to air rarefaction at an altitude of 5500m. Control animals were kept in the same room near the chamber at normal atmospheric pressure. The changes observed in the lungs were evaluated basing on ultrastructural analysis by transmission electron microscopy. The present results indicate phasic character of the changes occurring within lung parenchyma, including the surfactant system-forming structures. In the early phase (3 days), destructive-exudative changes predominated (delamelation of type II pneumocytes; oedematous changes, damage to the alveolar lining layer, accumulation of alveolar macrophages). In the later phases, repair-proliferative processes were predominant (increased number of type II pneumocytes, focal intensification of fibroplasia). The changes within the surfactant system-forming structures were accompanied by the accumulation of granulocytes and monocytes (hypobaric conditions for 3 and 10 days) and blood platelets (hypobaric conditions for 30 days) in the vascular bed of the lungs. The pathomechanism of the changes observed in the surfactant system was discussed with regard to the vascular changes. The possibility of formation of blood platelets within the vascular system of the lungs was considered.