Any of a class of physiologically active substances present in many tissues, with effects such as vasodilation, vasoconstriction, stimulation of the smooth muscles of the bronchus or intestine, uterine stimulation; also involved in pain, inflammation, fever, allergic diarrhea, and dysmenorrhea. A potent hormone — similar in structure to an unsaturated fatty acid — that acts in extremely low concentrations on local target organs; first isolated from the prostate.