The endocannabinoid 2-arachidonoylglycerol (2-AG) plays a major role in many physiological processes, and its action is quickly terminated via enzymatic hydrolysis catalyzed by monoglyceride lipase (MGL). Regulating its endogenous level could offer therapeutic opportunities; however, few selective MGL inhibitors have been described so far. Here, we describe the synthesis of N-substituted maleimides and their pharmacological evaluation on the recombinant human fatty acid amide hydrolase (FAAH) and on the purified human MGL. A few N-arylmaleimides were previously described ( Saario , S. M. ; Salo , O. M. ; Nevalainen , T. ; Poso , A. ; Laitinen , J. T. ; Jarvinen , T. ; Niemi , R. Characterization of the Sulfhydryl-Sensitive Site in the Enzyme Responsible for Hydrolysis of 2-Arachidonoylglycerol in Rat Cerebellar Membranes . Chem. Biol. 2005 , 12 , 649 - 656 ) as MGL inhibitors, and along these lines, we present a new set of maleimide derivatives that showed low micromolar IC(50) and high selectivity toward MGL vs FAAH. Then, structure-activity relationships have been investigated and, for instance, 1-biphenyl-4-ylmethylmaleimide inhibits MGL with an IC(50) value of 790 nM. Furthermore, rapid dilution experiments reveal that these compounds act as irreversible inhibitors. In conclusion, N-substituted maleimides constitute a promising class of potent and selective MGL inhibitors.