Isoprostanes are produced by the non-enzymatic, free radical peroxidation of phospholipid-esterified arachidonic acid. They have been used as biomarkers of oxidative stress, but they also have been found to have potent biological activity. ent-8-iso-15(S)-Prostaglandin F2α (ent-8-iso-15(S)-PGF2α) is a potent vasoconstrictor of porcine retinal and brain microvessels with EC50 values of 15 and 24 nM, respectively. This isoprostane is about ten-fold more potent than 8-iso-PGF2α in a whole blood platelet aggregation inhibition assay.