Prostaglandins occur in many animal species and tissues. They are C20-carboxylic acids contining one five-membered ring, one keto group, two or three hydroxy groups, and one to three CC double bonds. Their structures were elucidated by chemical, radiochemical, and physical methods in combination with gas chromatography and mass spectrometry. The prostaglandins are biosynthesized from essential fatty acids. By their action smooth muscles are stimulated and lipid metabolism is affected.