Abstract
1 . Slices of rabbit spleen generate rabbit aorta contracting substance (RCS) and a prostaglandin of the E series when they are vibrated or stirred. 2 . The release of both substances is increased by arachidonic acid and inhibited by aspirin-like drugs. 3 . When the spleen effluent containing RCS and prostaglandin E is incubated for a further 3 min, the prostaglandin concentration increases and the RCS concentration declines. 4 . RCS may therefore be an unstable intermediate in the biosynthesis of prostaglandins.