(±)-WIN 55,212-2 is a potent aminoalkylindole cannabinoid (CB) receptor agonist with a Ki value of 62.3 and 3.3 nM for human recombinant central cannabinoid (CB1) and peripheral cannabinoid (CB2) receptors, respectively. In contrast, the enantiomer (-)-WIN 55,212-3 acts a partial inverse agonist at CB1 (pIC50 = 5.5) and as a competitive neutral antagonist of CB2, reversing the inverse agonism evoked by SR 144528 (pEC50 = 5.3). (+)-WIN 55,212 (mesylate) is a mixture of the two enantiomers, (+)-WIN 55,212-2 and (-)-WIN 55,212-3.