F2-isoprostanes are prostaglandin (PG)F2-like compounds that are produced in vivo as non-enzymatic products of free radical catalyzed peroxidation of arachidonic acid. One F2-isoprostane whose formation should be favored is 8-epi-PGF2 alpha. 8-Epi-PGF2 alpha has been shown to exert potent bioactivity but proof that it is formed in vivo is lacking. Evidence is now presented suggesting that 8-epi-PGF2 alpha is, in fact, formed in vivo by demonstrating that an endogenous F2-isoprostane with a retention time on capillary GC identical with that of 8-epi-PGF2 alpha co-chromatographs through four high resolving HPLC purification procedures with authentic radiolabelled 8-epi-PGF2 alpha.