1 A simple double-isotope assay for phospholipase A2 activity of perfused organs is described; Guinea-pig lungs perfused through the pulmonary circulation exhibit a low background enzyme activity. This activity is blocked by dexamethasone, betamethasone and hydrocortisone, mepacrine, procaine or chlorpromazine. Aspirin and indomethacin are without effect. 3 Mechanical trauma, antigen challenge or injections of bradykinin, rabbit aorta contracting substance-releasing factor (RCS-RF) or histamine increase "basal" phospholipase activity. The effect of these agents, except that of bradykinin, is blocked by dexamethasone or mepacrine. 4 The blocking effect of steroids is cumulative and dose-dependent. They do not work in cell-free systems. Inhibition by mepacrine is rapid and is effective in cell-free lung homogenates. 5 It is suggested that agents which liberate prostaglandin endoperoxides and thromboxane A2 from perfused lungs do so by activating phospholipase A2.