AbstractThe suprachiasmatic nucleus (SCN) has been proposed as a site for an endogenous circadian oscillator in mammals, since lesions of the nucleus abolish a wide spectrum of overt circadian rhythms. To demonstrate that a directly measurable property of the SCN itself in intact (unlesioned) animals is affected by environmental light and exhibits circadian rhythmicity, we used the autoradiographic 2‐deoxy‐D‐[14C]glucose method to determine glucose utilization of rat SCN under a variety of lighting conditions. Our experiments indicate an important role for the SCN in circadian rhythm organization, and we believe the deoxyglucose method will prove useful as a tool for better understanding the functions and mechanisms of circadian c