1. Rabbit aorta contracting substance (RCS) and prostaglandins were released from guinea-pig isolated perfused lungs by gentle massage and also by infusion of Prosparol.2. RCS and prostaglandins were also released by infusion into the pulmonary artery of bradykinin, arachidonic and dihomo-gamma-linolenic acids or shock perfusate (containing RCS-releasing factor).3. Arachidonic and dihomo-gamma-linolenic acids caused a prolonged release of RCS and prostaglandins whereas bradykinin and shock perfusate gave a short-lasting output.4. RCS and prostaglandins, together with histamine were released when superfused chopped lung tissue was agitated.5. Challenge of sensitized guinea-pigs in vivo led to the release of an RCS-like substance into the carotid arterial blood.6. Intravenous injection of bradykinin into guinea-pigs also released an RCS-like substance.7. The release of RCS and prostaglandins was inhibited by aspirin or indomethacin in all experiments.8. RCS contracted all vascular tissues investigated and also rat stomach strip.9. The half-life of RCS was estimated as 1-2 minutes.