Heneicosapentaenoic Acid (HPA) is a 21:5 ω-3 fatty acid present in trace amounts in the green alga B. pennata and in fish oils. Its chemical composition is similar to eicosapentaenoic acid (EPA) except elongated with one carbon on the carboxyl end, placing the first double bond in the δ6 position. HPA can be used to study the significance of the position of the double bonds in ω-3 fatty acids. It incorporates into phospholipids and into triacylglycerol in vivo with the same efficiency as EPA and docosahexaenoic acid and exhibits strong inhibition of arachidonic acid synthesis from linoleic acid. HPA is a poor substrate for prostaglandin H synthase (PGHS) (cyclooxygenase) and for 5-lipoxygenase but retains the ability to rapidly inactivate PGHS.