The generation from arachidonic acid of rabbit aorta contracting substance (RCS) by a microsomal enzyme preparation which also generates prostaglandins.

1. Sodium arachidonate was incubated with a crude prostaglandin synthetase preparation made from dog spleen. The incubation was made in a dynamic system so that the products could be delivered to strips of rabbit aorta and rat stomach.2. A rabbit aorta contracting substance (RCS) and a prostaglandin-like substance were formed. The RCS had similar properties to that released in anaphylaxis. It was unstable and as the RCS activity declined, so the prostaglandin-like activity increased. Its formation was prevented by prostaglandin synthetase inhibitors such as indomethacin.3. A different rabbit aorta contracting substance was formed by incubation of arachidonate with lipoxygenase, which generates peroxides. This substance was stable and did not lead to prostaglandin production.4. We conclude that RCS may be an intermediate in prostaglandin production, such as the postulated cyclic endoperoxide.