Heneicosapentaenoic Acid (HPA), a 21:5 ω-3 fatty acid, is found in minute quantities in green algae and fish oils, resembling eicosapentaenoic acid (EPA) but with an added carbon on the carboxyl end, positioning the initial double bond at the Δ6 location. HPA serves as a tool for examining the impact of double bond positions within n-3 fatty acids, as it is incorporated into phospholipids and triacylglycerol in vivo as efficiently as EPA and docosahexaenoic acid (DHA), while significantly inhibiting the synthesis of arachidonic acid from linoleic acid. Moreover, the ethyl ester variant of heneicosapentaenoic acid offers a more lipophilic and stable alternative to the free acid form.