(±)13-HDHA is an autoxidation product of docosahexaenoic acid (DHA) in vitro. It is also produced from incubations of DHA in rat liver, brain, and intestinal microsomes. Fresh water hydra was shown to metabolize DHA to 13(R)-HDHA, presumably via the 11R-lipoxygenase activity. (±)13-HDHA is a potential marker of oxidative stress in brain and retina where DHA is an abundant polyunsaturated fatty acid.