The circadian timing system imposes a temporal organization on physiological processes and behavior. The two major nuclei of the system are the intergeniculate leaflet (IGL) of the lateral geniculate complex and the suprachiasmatic nucleus (SCN) of the hypothalamus. In this study, we demonstrate that neurons of both nuclei colocalize GABA with peptides. In the IGL, GABA is colocalized with neuropeptide Y in neurons projecting to the SCN and with enkephalin in neurons projecting to the contralateral IGL. In the SCN, GABA is colocalized with vasopressin and vasoactive intestinal polypeptide. All, or nearly all, of the neurons in the IGL and SCN are GABA-producing. Thus, GABA should be considered the principal neurotransmitter of the circadian system.