Toxic oxygen metabolites (TOM) have been suggested to be mediators of permeability edema associated with the adult respiratory distress syndrome (ARDS). Because corticosteroids have possible beneficial effects in ARDS, we have examined the effect of methylprednisolone (MP) on TOM-induced lung edema in isolated, plasma-perfused rat lungs. TOM were generated by adding xanthine oxidase (XO) and hypoxanthine (HX) to the perfusate. Microvascular permeability was assessed by fluid filtration rate (FFR). FFR was determined before and 30 min after administration of XO and HX by measuring the weight increase of the lungs for the last 3 min during a standard 5 min elevation of the outlet pressure. MP was administered in two different ways: 1) Added to the perfusate 5 or 60 min before XO and HX (0.1 and 1 mg ml-1), and 2) given as pretreatment to the rats 12 and 2 hr before preparation of the lungs (40 mg kg -1). XO and HX significantly increased FFR compared to lungs perfused with untreated plasma. Pretreatment with MP significantly attenuated the increase in FFR caused by XO and HX. Addition of MP to the perfusate also inhibited the effect of TOM. This latter protection occurred irrespective of when MP was added before XO and HX. However, when the highest dose of MP was added 5 min before XO and HX, there was a loss of the protective effect. In summary, this study provides evidence that MP may directly prevent microvascular injury induced by TOM in isolated perfused rat lungs. The effect was dependent on the dose of MP applied, but not on when MP was administered prior to exposure to TOM.