The metabolism of the double bonds at the A3 position in fatty acids was studied in rat liver. Infusion of A3-trans-dodecenoic acid into isolated perfused liver and subcellular fractionation studies showed the presence of both peroxisomal and mitochondrial A3,A2-enoyl-CoA isomerase activity (EC 5.3.3.8). These findings together with the previous demonstration of peroxisomal 2,Cdienoyl-CoA reductase (EC 1.3.1.34) [(1981 )