The aim of this study was to determine the effects of circulating catecholamines and light on the daily melatonin rhythm in the marmot. Endogenous levels of circulating catecholamines and plasma melatonin were measured during arousal from hibernation in light and stuthes were performed on the circadian melatonin rhythm in two photoperiods (LD 4:20 and LD 8:16). In addition, stuthes were done on the capacity of broad-band white light at normal room intensities (32 μm2 or 108 lx) and of low-intensity monochromatic green light (500 nm; 1.4 μm2 or 31 lx) to suppress high nocturnal melatonin levels. We conclude that high levels of plasma catecholamines that occur during arousal from hibernation do not influence the production and secretion of pineal melatonin. During the nocturnal portion of its light/dark cycle, the marmot plasma melatonin rhythm is suppressed by both white light and low-intensity green light.