N-arachidonoyl-glycine (NAGly) has been recently identified in rodent tissues and found to exhibit analgesic activity in vivo. NAGly is a potent inhibitor of the fatty acid amide hydrolase (FAAH), the enzyme primarily responsible for the degradation of the endocannabinoid N-arachidonoyl-ethanolamine (anandamide), and was shown recently to elevate the blood levels of the this analgesic compound. We have synthesized several N-arachidonoyl-amino acids of potential natural occurrence, as well as the D- and L-isomers of N-arachidonoyl-alanine, and have tested their activity on FAAH preparations from mouse, rat, and human cell lines, and from mouse or rat brain. The results indicate that the relative potency and enantioselectivity of N-arachidonoyl-amino acids as FAAH inhibitors depend on the animal species. Thus, whilst NAGly is the most potent compound on the rat and mouse enzymes, N-arachidonoyl-isoleucine is active only on human FAAH and N-arachidonoyl-alanine enantiomers show a varying degree of potency. Taken together, these data support the view that an enhancement of endogenous anandamide levels underlies in part the analgesic effects of NAGly in rodents.