Melatonin has been found to inhibit or enhance the constitutive secretion of proteins from the cultured melanoma cells at nanomolar concentrations (0.5-10 nM), in a dose dependent manner. The amplitude and direction of the response were found to depend on cell density: melatonin inhibited the release early after plating or at low cell density, but facilitated the release later on, or at high cell density. To elucidate the involvement of G-proteins in these responses, the effects of guanosine 5'-O-(3-thiotriphosphate) (GTP tau S; which was introduced into the cells during the process of permeabilization and resealing with ATP), aluminum fluoride, pertussis and cholera toxins on protein secretion from the cells were assessed in the absence and presence of melatonin. At low cell density, melatonin inhibited release, but paradoxically enhanced it when GTP hydrolysis was blocked (by GTP tau S or cholera toxin treatment). Aluminum fluoride and melatonin inhibited protein release in the absence or presence of GTP tau S. At high cell density, melatonin facilitated the release and so did GTP tau S, aluminum fluoride, their combination, and cholera toxin treatment. However, in the presence of the combination of GTP tau S, aluminium fluoride and melatonin, protein release was paradoxically inhibited. Similar treatment of the cells with pertussis toxin, did not affect the melatonin-mediated inhibition or facilitation. These results indicate that the effects of melatonin on protein secretion are mediated by at least one heterotrimeric G protein which belongs to the Gs class. In addition, melatonin can facilitate secretion via a cholera and pertussis toxins-insensitive mechanism which can be inhibited by aluminum fluoride. This effect is manifested when Gs is permanently activated (by GTP tau S or cholera toxin).