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Daily profiles of the extracellular concentrations of glutamate (GLU), aspartate, and glutamine in the region of the suprachiasmatic nuclei (SCN) of the Djungarian hamster were examined by in vivomicrodialysis. Hamsters with a dialysis probe located in or near the SCN exhibited a diurnal variation in the extracellular concentration of GLU. Glutamate levels rose gradually during the latter half of the subjective day and through the night to reach peak levels near the time of lights-on. The extracellular concentration of GLU was significantly elevated (P< 0.05) during the latter half of the dark phase relative to the level at midday. The average peak in extracellular GLU was 217% of the midday value (P< 0.05; n= 5). Diurnal fluctuations were not consistently observed in the concentrations of aspartate in SCN dialysates. The nocturnal rise in extracellular GLU in the SCN was not attenuated by perfusion with 10 μMtetrodotoxin, although this concentration of tetrodotoxin completely blocked increases in extracellular GLU caused by simultaneous perfusion with 30 μMveratridine. Collectively, these results point to a diurnal rhythm in extracellular concentration of GLU in the SCN region which may not reflect diurnal variations in synaptic activity.