Anatomical techniques have shown that the suprachiasmatic nuclei (SCN) are organized into distinct dorsomedial and ventrolateral subdivisions. As a functional correlate to this morphological organization, the intranuclear distribution of SCN glucose utilization was mapped using the autoradiographic 14C-labeled deoxyglucose method. In nocturnal rats and diurnal squirrel monkeys injected with the tracer during the light portion of the light-dark cycle, the middle of the SCN was metabolically more active than its rostral or caudal ends. No obvious dorsomedial/ventrolateral parcellation of SCN functional activity was disclosed. The rostrocaudal metabolic contour persisted unchanged in the absence of external light and resembled the 3-dimensional shape of the SCN (the highest metabolic activity was generally found at the largest cross-sectional area). This result is discussed with respect to its implications for the generation of circadian rhythmicity by the endogenous pacemaker in the SCN.