Melatonin is an essential component for circadian system function, whose daily plasma secretory rhythm is driven by the suprachiasmatic nucleus (SCN), contributing to the communication of temporal messages from the central circadian clock to all cells. Melatonin secretion peaks in the dark, regardless of whether animals are diurnal or nocturnal. To date, the precise mechanisms that explain how the circadian system is configured as nocturnal or diurnal remain unknown. The present study examines mid-day and midnight melatonin plasma levels and the influence of exogenous melatonin on the circadian system phasing of Octodon degus, a diurnal rodent, which exhibits nocturnal and diurnal chronotypes when free access to a wheel is provided. Plasma levels of melatonin were determined by RIA in blood samples taken from the jugular vein at mid-light (ML) and mid-dark (MD). Melatonin (0.5 mg/kg b.wt.) was orally administered in their drinking water for 30 days, 2 hr before the onset of darkness. The results showed that plasma melatonin levels and their qualitative effects, hypothermia and improved synchronization with no modification in the 24-hr wheel running activity (WR), were similar in both nocturnal and diurnal degus. Furthermore, melatonin can be used to improve the impaired circadian rhythmicity observed in aged animals, with no rebound effect after ceasing the treatment. It is concluded that plasma melatonin levels and the differential responses to melatonin do not seem to be responsible for nocturnal and diurnal chronotypes, and thus other mechanisms upstream, within, or downstream from the SCN should be investigated.