Homogenates of the lamb ductus arteriosus at term have the capacity to convert exogenous arachidonic acid into prostaglandins E2, F2alpha and 6-ketoprostaglandin F1alpha. No evidence was obtained for the formation of prostaglandin D2 and thromboxane B2. All products were quantitated by mass spectrometry. 6-Ketoprostaglandin F1alpha was the major compound, formed in amounts in excess of 10 fold over prostalgandins E2 and F2alpha.