The interferon-gamma receptor gene was detected in the rat hypothalamic suprachiasmatic nuclei (SCN), the main pacemaker for circadian rhythms, and the molecular identity of the transcript was confirmed by sequencing. The expression of the receptor protein showed a daily rhythm that was dependent on light. It reached its adult pattern in the SCN between postnatal day 11 and 20, i.e., at a time when capacity for photic entrainment of the pacemaker is established.