The influence of 12-, 24- and 48-h normobaric hyperoxia on phospholipid composition of lung surfactant in rats was investigated. Both in surfactant isolated from bronchoalveolar lavage and in surfactant isolated from lung tissue after 12 h breathing of pure oxygen, a decrease in both phosphatidylcholine (particularly in disaturated ones) and phosphatidylglycerol content was observed. This decrease was compensated with concomitant increase in lysophosphoglyceride content. A drop in total phospholipid level in surfactant of oxygen-exposed rats was observed. These data suggest that normobaric hyperoxia causes a substantial changes in lung surfactant composition and type II pneumocyte metabolism.