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 structure is similar to eicosapentaenoic acid (EPA) but elongated with one carbon at the carboxyl end, placing the first double bond in the δ6 position. HPA is utilized to study the significance of the position of double bonds in ω-3 fatty acids and incorporates into phospholipids and triacylglycerol in vivo as efficiently as EPA and docosahexaenoic acid. It strongly inhibits arachidonic acid synthesis from linoleic acid, is a poor substrate for prostaglandin H synthase (PGHS) (cyclooxygenase) and 5-lipoxygenase, but rapidly inactivates PGHS.