Arachidonic acid is the keystone essential fatty acid at the origin of the arachidonic acid cascade. It is converted by cyclooxygenase, lipoxygenase, and epoxygenase enzymes into more than one hundred fifty different potent primary autacoid metabolites in species ranging from fungi to plants to mammals. Arachidonic acid is stored in tissue phospholipids in esterified form, where it comprises a small but critically controlled percentage of the PUFA pool. Arachidonic acid content is frequently measured by the saponification of the lipid fraction followed by methyl esterification and gas chromatographic analysis of the resulting FAME compounds. Arachidonic acid methyl ester can also be incorporated into dietary regimens or fed to cultured cells as a source of exogenous arachidonate.