Nasopharyngeal carcinoma (NPC) occurs frequently in southern China. The circadian rhythm of DNA synthesis of a poorly differentiated NPC human cell line (CNE2) was investigated as an experimental prerequisite for designing chrono-chemotherapy schedules for patients with this disease. Twenty-two nude mice with BALB/c background were synchronized alternatively in 12h of light and 12h of darkness (LD12:12) for at least 3wk prior to the transplantation of a CNE2 tumor fragment into each flank (area of approximately 2 x 2 mm2). Ten days later, a tumor sample (area of approximately 5 mm2) was obtained at 3, 9, 15, and 21 h after light onset (HALO) alternatively from different sites in each mouse. Single-cell suspensions were prepared and stained with propidium iodide. Cellular DNA content was measured with flow cytometry. Data were analyzed by ANOVA and cosinor methods. The average proportion of tumor cells in G1, S or G2-M phase varied according to circadian time with statistical significance. The maximum occurred at 9 HALO for G1, 2 HALO for S and 21 HALO for G2-M phase cells. The approximate average distribution patterns of G1 and G2-M phases of cosine curve was 24 h. This was not the case for S-phase cells, which displayed a bimodal temporal pattern. Inter-individual variability in peak time was large, possibly due to relatively sparse sampling time. Nevertheless, no more than 6% of the time series displayed a maximum at 3 HALO for G1, 21 HALO for S and 15 HALO for G2-M. The cell cycle distribution of this human NPC cell line displayed circadian regulation following implantation into nude mice. The mechanisms involved in this rhythm and its relevance to the chrono-chemotherapy of patients deserve further investigation.