Using in situ hybridization, we examined temporal changes of the EAAC1 glutamate transporter mRNA within the suprachiasmatic nuclei (SCN) of rats in constant darkness. Film autoradiographs showed that the SCN and supraoptic nuclei (SON) contained a marked density of hybridization signal. Analysis of silver grains per cell in emulsion-dipped sections indicated that cellular expression of EEAC1 mRNA in the SCN was elevated during the latter part of the subjective night and at the beginning of the subjective day, with a peak at circadian time 23.1 as determined by cosinor analysis. The times at which EAAC1 mRNA is highest correspond to the time points at which extracellular glutamate, a neurotransmitter that putatively mediates photic entrainment, has been reported to be low within the SCN. The presence of EAAC1 mRNA in the SCN and SON may partially explain the resistance of these nuclei to glutamate receptor-mediated excitotoxins; furthermore, the raised level preceding subjective dawn in the SCN may ensure sub-toxic levels of extracellular glutamate at the onset of photic stimulation during the LD cycle. In contrast, cellular expression of EAAC1 mRNA in the cingulate cortex and reticular thalamus remained constant at all time points studied. These results suggest that there is circadian control of the EAAC1 mRNA by the clock intrinsic to the SCN.