Northern blot analysis revealed the circadian expression of rat period homologue (rPer2) mRNA in peripheral mononuclear leukocytes that have no neuronal connections. The mRNA expression in the suprachiasmatic nucleus (SCN), peripheral mononuclear leukocytes, and other peripheral tissues exhibited prominent, synchronous circadian oscillation with peaks at early night. As we have previously shown the peripheral circadian expression of rPer2 mRNA is abolished in SCN-lesioned rats [Sakamoto, K., Nagase, T. Fukui, H., Horikawa, K., Okada, T., Tanaka, H., Sato, K., Miyake, Y., Ohara, O., Kako, K. and Ishida, N., J. Biol. Chem., 273 (1998) 27039–27042], our findings suggest that some humoral signals, driven by the SCN, mediate the circadian expression of mammalian per homologues in peripheral tissues.