Arachidonic acid and ‘slow-reacting substance C, perfused through isolated guinea-pig lungs, release a ‘rabbit aorta contracting substance C (RCS-C), indistinguishable from a RCS released under similar conditions by anaphylaxis, bradykinin and ‘slow-reacting substance A’. Release of RCS-C is blocked by previous perfusion of the lungs with aspirin or phenylbutazone. A role for ‘rabbit aorta contracting substances’, released during inflammation by a cascade of events initially evoked by phospholipase A activation, is suggested.