Prostaglandin F2α ethanolamide (PGF2α-EA-d4) contains four deuterium atoms at the 3, 3', 4, and 4' positions. It is intended for use as an internal standard for the quantification of PGF2α-EA by GC- or LC-mass spectrometry. PGF2α-EA is produced by cyclooxygenase 2 (COX-2) metabolism of the endogenous cannabinoid arachidonoyl ethanolamide (AEA) found in brain, liver, and other mammalian tissues.¹ AEA can be metabolized by fatty acid amide hydrolase (FAAH) to give free arachidonic acid, which is the well known and conventional substrate for COX enzymes. However, it has also been reported that AEA can be used directly by COX-2 to produce ethanolamide congeners of the classical prostaglandins, including PGE2.2 PGF2α-EA has also been reported to be biosynthesized by this mechanism when AEA was infused into the lung and liver of living mice. PGF2α-EA is a potent dilator (EC50 = 58 nM) of the cat iris sphincter, which is a model system for testing potential intraocular hypotensive agents.³