THE suprachiasmatic nuclei serve as the dominant circadian pacemaker in the mammalian brain, regulating daily behavioral, physiological and hormonal rhythms. In the ventrolateral parts of these nuclei, the receptor for the key immunoregulatory molecule interferon-γ (IFN-γ) was detected in the rat brain. The cellular localization of the IFN-γ receptor corresponded to neuronal elements. Expression of the receptor followed a diurnal rhythm with a peak at zeitgeber time 15. This peak coincided with an enhanced expression of janus kinase 1 and 2 as well as the signal transducer and activator of transcription 1, which constitute the main intracellular signaling pathway of IFN-γ. This is the first study to show expression of the receptor of an immune modulatory molecule in the pacemaker of the biological clock, which, thus, may be influenced by immune system signal molecules.