(R)-AM1241 binds to cannabinoid (CB) receptors and has greater than 100-fold selectivity for the CB2 over the CB1 receptor (Kis = 15 and 5,000 nM, respectively, in a membrane assay using human receptors). (R)-AM1241 acts as an agonist at human CB2 receptors (EC50 = 118 nM) but an inverse agonist at rat and mouse CB2 receptors (EC50s = 315 and 341 nM, respectively). Similar to the racemate AM1241 , (R)-AM1241 produces antinociception to thermal, but not mechanical, pain in rats. The pain-reducing effect of (R)-AM1241 is blocked by the CB2-specific inhibitor SR 144528 but not by either the CB1-selective inhibitor rimonabant or the opioid receptor blocker naloxone .