Circadian effects of exogenous melatonin, whereby daily administration induces entrainment or phase shifts, have been demonstrated in both nocturnal and diurnal mammals. In Long-Evans rats, as used in early studies, effects occur reliably when melatonin is administered late in the subjective day. A second period of sensitivity to melatonin, late in the subjective night, is evident in certain strains of mice and the diurnal Funambulus pennanti. This late night to early morning sensitive phase previously had been identified in human subjects. Different circadian responses to melatonin also may occur between rat strains. Circadian effects of melatonin in hamsters are diverse and vary with strain, developmental age, and method of administration. Characteristics of melatonin binding sites within the suprachiasmatic nuclei vary both between and within species, as do profiles of endogenous melatonin rhythms. These differences may explain the variations in circadian responses to melatonin.