Contents of adenosine triphosphate (ATP) in the suprachiasmatic nucleus (SCN), anterior hypothalamic area (AH) and caudate putamen (CPu) were determined every 4 h in rats kept under constant dark conditions. All tested regions of the brain exhibited circadian rhythms in ATP content. Among these, the amplitude of the rhythm was largest in the SCN. Peaks of the ATP content were observed at circadian time 18 in the SCN, 6 in the AH and 10 in the CPu. In view of the observation that at around these peak times electrical or glucose uptake activity of each brain region was lowest, the present result suggests that ATP content is negatively correlated with electrical or glucose uptake activity.