A good model of adult respiratory distress syndrome is lung injury induced by phorbol myristate acetate (PMA). In the present study we examined the effect of mepacrine, an inhibitor of phospholipase A2, on lung injury induced by PMA in isolated blood-perfused rat lungs. In the isolated lung, saline (1 ml) or mepacrine (75 microM) alone in the perfusion system did not discernibly change the pulmonary arterial pressure (PAP) and lung weight (LW). After administration of PMA (0.16 micrograms/ml), severe hypertension and lung edema developed (delta PAP = 40.1 +/- 6.0 mmHg, p less than 0.001; delta LW = 5.5 +/- 0.7 g, p less than 0.001). Whereas, the addition of mepacrine (75 microM) prevented PMA-induced lung edema and pulmonary hypertension (delta PAP = 4.7 +/- 2.2 mmHg, delta LW = 0.2 +/- 0.2 g). To further elucidate the protective mechanism of mepacrine on lung injury, a vasodilator (nitroprusside) was given to decrease PAP levels to +6 mmHg from baseline values in the PMA group, as well as in the mepacrine-pretreated PMA (MPMA) group. During a subsequent venous pressure challenge, severe lung injury developed in the PMA group (delta LW = 9.5 +/- 2.1 g, p less than 0.001). However, with the same venous pressure challenge in the MPMA the lung weight was markedly less than that of the PMA group (delta LW = 1.0 +/- 0.2 g). Histologic findings examined by light microscopy presented intraalveolar hemorrhage and fluid accumulation, disruption of vascular basements and alveolar septa, and aggregation of inflammatory cells within the parenchyma in the lungs of the PMA group. In the MPMA group there was no evidence of intraalveolar hemorrhage and alveolar fluid accumulation, however, the occasional presence of granulocytes in the parenchyma and slight interstitial edema were still observed. In addition, depressed the chemiluminescence release from PMA activated granulocytes which were in a dose-dependent manner in vitro. These observations suggest that mepacrine inhibits PMA-induced lung injury chiefly by protection of vascular permeability. The mechanism of the protection may be due to the inhibition of oxygen radicals released from activated neutrophils and the reduction of neutrophil chemotaxis.