Pulmonary diffusion capacity (DLCO) is reduced 2 h after various types of exercise, such as rowing, treadmill running, arm cranking and marathon running. The decrease in DLCO may involve alterations in the alveolar-capillary membrane as well as depletion of the central blood volume. We hypothesized that the reduction in DLCO might also be influenced by oxygen free radicals, acute phase proteins and endotoxin, which are also involved in the adult respiratory distress syndrome (ARDS). Ten competitive male oarsmen performed a 6 min 'all-out' ergometer row. Single breath DLCO was determined before and 2 h after rowing and venous blood samples were also obtained during the row. Absolute DLCO decreased by 11% (range 0-20%) 2 h after rowing, whereas the concentration of endotoxin did not change significantly and interleukin (IL)-1-alpha, IL-8 and tumour necrosis factor (TNF)-alpha were below the levels of detection before, during and 2 h after rowing. Oxygen free radicals were evaluated by oxidative modification of amino acids and DNA. Corrected for creatinine in urine voided 3 h post-exercise, the DNA repair product 8-oxo-7,8-dehydro-2-deoxyguanosine (8-oxodG) did not change significantly. The ratio of fluorescence due to dityrosine to that due to tryptophan in plasma proteins increased after exercise. This might reflect an effect of oxygen free radicals, but it might also indicate an altered relative composition of plasma proteins. These results suggest that the reduced pulmonary diffusion capacity following exercise is unrelated to factors typically associated with ARDS.