In agreement with the highly lipophilic nature of (-)-delta9-tetrahydrocannabinol, all the endogenous ligands of cannabinoid receptors identified so far are derivatives of long chain fatty acids. N- Arachidonoylethanolamine (anandamide) and some of its polyunsaturated congeners have been found in mammalian brain and shown to activate the CB1 and, with a lower efficacy, CB2 cannabinoid receptor subtypes. More recently, 2-arachidonoylglycerol (2-AG), a widespread intermediate in the metabolism of phosphoglycerides, diacylglycerols and triglycerides, was also found to activate the cannabinoid receptors. The capability of palmitoylethanolamide, an anti-inflammatory metabolite, to activate CB2-like receptors is still being debated. Here we review: 1) the metabolic pathways suggested so far to underlie the biosynthesis and inactivation of anandamide and 2-AG, and 2) the current knowledge of the chemical bases for the interactions of anandamide and 2-AG with proteins of the endogenous cannabinoid system characterized so far, i.e. the CB1 and CB2 receptor subtypes, the membrane anandamide carrier , which facilitates anandamide diffusion into cells, and the enzyme fatty acid amide hydrolase , which catalyzes anandamide and, to a certain extent, 2-AG hydrolysis in vivo.