Increasing amounts of data have indicated the physiological significance of circadian clock gene regulation in various peripheral cells. In the present study, we examined expression of the human homolog of period1 (hPer1) in peripheral mononuclear cells (MNCs) and polymorphonuclear neutrophils (PMNs) in seven healthy young male volunteers (mean age, 21.0 years; range, 19-24 years) under modified constant routine conditions. The expression of hPer1 as determined by real-time PCR with gene-specific hybriprobes in MNCs and PMNs showed significant daily variations with similar acrophases and peak transcription in the subjective morning. The acrophases in hPer1 expression rhythms in MNCs and PMNs were found to correlate positively with that of the serum melatonin secretion rhythms, which is a reliable phase marker of the suprachiasmatic nucleus (SCN), the circadian master clock. The present findings indicate that clock gene activity could be preserved across different peripheral blood cell types and support the assumption that peripheral clocks are entrained by the SCN.