Objective: To determine whether there exists melatonin receptor (MR) in the mouse spleen and the circadian rhythm (CR) in MR. Methods: Specific binding of melatonin to the mouse spleen was measured by radioligand binding assay. Results: The results revealed the maximum binding capacity (Bmax) was (1.25±0.21) fmol/mg protein and equilibrium constant (Kd)was (59.5±10.1) pmol/L. The peak values of Bmax of MR was from 12:00 to16:00 and the trough values was between 0:00 to 4:00. The difference between them was significant statistically. CR of MR showed no change when continued light (3 weeks) and injected melatonin in peritoneum (3 weeks). Conclusion: MR is present in the mouse spleen and there exists CR of MR in the mouse spleen. It is indicated that CR of MR in spleen is independent from that of melatonin.