The Xenopus melatonin receptor was expressed in human embryonic kidney 293 cells and assayed for cAMP accumulation. In transfected 293 cells expressing the melatonin receptor, melatonin dose-dependently inhibited the endogenous adenylyl cyclases. In contrast, melatonin stimulated the accumulation of cAMP in cells co-expressing the type II adenylyl cyclase. Both the inhibitory and stimulatory responses to melatonin were mediated via G i -like proteins as they were blocked by pertussis toxin. Upon co-transfection with the α subunit of G z , the ability of melatonin to regulate both type II and the endogenous adenylyl cyclases became refractory to pertussis toxin, indicating that the melatonin receptor can also couple to G z . However, other pertussis toxin-insensitive G proteins such as G q , G 1 2 and G 1 3 were unable to interact with the melatonin receptor.