A control group of rats was exposed to filtered air and an experimental group to nitrogen dioxide levels of 15 ppm. Surface tension-area curves were recorded from fresh lung washings of excised lungs. Total phospholipid and lecithin concentrations were determined as an index of surfactant quantity, and an analysis of the surface tension-area curves was made by computer techniques. Total phospholipid and lecithin concentrations from the experimental animals were significantly higher, but the percentage contributed by lecithin was nearly identical for both groups. Nitrogen dioxide exposure at this level appears to alter the properties of surfactant demonstrated on surface tension balance, since higher trough concentrations of the experimental wash produced nearly normal surface tension-area loops. Changing surface tension properties of surfactant may be important in nitrogen dioxide-induced pulmonary injury.