Arachidonoyl ethanolamide-d4 (AEA-d4) contains four deuterium atoms at the hydroxyethyl 1,1',2, and 2' positions. It is intended for use as an internal standard for the quantification of AEA by GC- or LC-mass spectrometry. AEA is the ethanolamine amide of arachidonic acid, first isolated from porcine brain.{1134} It is an endogenous cannabinoid neurotransmitter that binds to both central cannabinoid (CB1) and peripheral cannabinoid (CB2) receptors and mimics the pharmacologic effects of ?9-THC.{2713} AEA inhibits the specific binding of [3H]-HU-243 to synaptosomal membranes with a Ki value of 52 nM, compared to 46 nM for ?9-THC.{1134}