As the data on circadian variations in plasma glucose or insulin are rather controversial due to interactions with food intake, this work attempted to characterize more precisely the daily rhythm of plasma glucose, insulin, growth hormone and melatonin in rats and to determine whether hormone rhythms occur independently of glucose variations. Plasma glucose, insulin, growth hormone and melatonin were investigated in rats infused for 24 h with a saline (fasted rats) or glucose solution (hyperglycaemic rats). Samples were taken every 2 h during a 24-h period. In fasted rats, both a glucose and an insulin diurnal rhythm were observed. The glucose rhythm was mainly characterized by an increase at the beginning of the night period, similar to that of the dawn phenomenon in humans but in opposite circumstances. Insulin rhythm appeared to be independent of glucose variations as it was still observed in rats maintained in stable hyperglycaemia (13.5 mM). A nycthemeral rhythmicity of growth hormone was observed in fasted and hyperglycaemic rats, with higher fluctuating values during the day period. As expected, plasma melatonin levels were characterized by a rise during the night period in both groups, although the rise was shifted in hyperglycaemic as compared to fasted rats. The main results of this study are the presence of an insulin secretion rhythmicity independent of glucose variations and the existence of a diurnal plasma glucose rhythm, with an increase occurring at the beginning of the night in fasted rats.