Summary Melon tissues from the porpoise ( Tursiops truncatus ) were incubated with L-(U-14C) leucine in Krebs-Henseleit buffer solution fortified with glucose. Rapid decarboxylation of the radioactively labelled L-leucine was accompanied by extensive incorporation of radioactivity into isovaleroyl chains of triacylglycerols. Relatively little radioactivity was incorporated into long-chain acids. The findings show that isovaleryl CoA, a catabolite of L-leucine, is directly incorporated into lipid structure to a significant degree instead of being degraded to acetate and acetoacetate by the pathway characteristic of other mammalian tissues.