(±)11-HDHA is an autoxidation product of DHA in vitro. It is also produced from incubations of DHA in rat liver, brain, and intestinal microsomes. DHA is metabolized to 11(S)-HDHA by human platelets and canine retina. In addition to 11(S)-HDHA, 14(S)-HDHA is also produced by platelets. 11(S)-HDHA is an inhibitor of U-46619-induced human platelet aggregation and rabbit and rat aortic smooth muscle contraction with IC50 values of about 50, 4.7, and 7.5 µM, respectively. (±)11-HDHA is a potential marker of oxidative stress in brain and retina where DHA is an abundant PUFA.