Melatonin has been identified in a variety of crustacean species, but its function is not as well understood as in vertebrates. The present study investigates whether melatonin has an effect on crustacean hyperglycemic hormone (CHH) gene expression, oxygen consumption (VO2) and circulating glucose and lactate levels, in response to different dissolved-oxygen concentrations, in the crab Neohelice granulata, as well as whether these possible effects are eyestalk-or receptor-dependent. Melatonin decreased CHH expression in crabs exposed for 45 min to 6 (2, 200 or 20,000 pmol.crab(-1)) or 2 mgO(2).L-1 (200 pmol.crab(-1)). Since luzindole (200 nmol.crab(-1)) did not significantly (p > 0.05) alter the melatonin effect, its action does not seem to be mediated by vertebrate-typical MT1 and MT2 receptors. Melatonin (200 pmol.crab(-1)) increased the levels of glucose and lactate in crabs exposed to 6 mgO(2).L-1, and luzindole (200 nmol.crab(-1)) decreased this effect, indicating that melatonin receptors are involved in hyperglycemia and lactemia. Melatonin showed no effect on VO2. Interestingly, in vitro incubation of eyestalk ganglia for 45 min at 0.7 mgO(2).L-1 significantly (p < 0.05) increased melatonin production in this organ. In addition, injections of melatonin significantly increased the levels of circulating melatonin in crabs exposed for 45 min to 6 (200 or 20,000 pmol center dot crab(-1)), 2 (200 and 20,000 pmol center dot crab(-1)) and 0.7 (200 or 20,000 pmol center dot crab(-1)) mgO(2).L-1. Therefore, melatonin seems to have an effect on the metabolism of N. granulata. This molecule inhibited the gene expression of CHH and caused an eyestalk-and receptor-dependent hyperglycemia, which suggests that melatonin may have a signaling role in metabolic regulation in this crab.