SO2 exposure (50-500 ppm) of isolated, perfused and ventilated guinea pig lungs, via the air passages, caused a concentration-related reduction in dynamic compliance and conductance. No changes in pulmonary perfusion flow was noted at any SO2 concentration. Formed sulfite was detected in lung lavage fluid as well as in the perfusate. Pretreatment of the lungs with a low concentration of SO2 (10 ppm) for 30 min protected against bronchoconstriction by a high concentration of SO2 (250 ppm). A similar protective effect was noted by pretreatment with sodium sulfite (3 mM) in the lung perfusate.