Abstract： Environmental stress (ES) is commonly used in producing chronic unpredictable mild stress to study pathogenesis of depression,including the regulatory role of circadian system on depression.However,the direct effect of ES on the circadian system has been rarely explored.The present study was aimed to investigate the effect of ES on depression-like behaviors and diurnal rhythm of plasma hormone/peptide levels in male rats.Rats were allocated into control group (CON group),low frequency ES group (LF group) and high frequency ES group (HF group).Sucrose preference test (SPT),open field test (OFT),weight gain,food and water intake were conducted to assess depression-and anxiety-like behaviors.A total of 7 times of the tail venous blood was collected with an interval of 4 h during 24 h from other rats who subjected to the same procedures of ES but not the behavioral tests.The alterations of diurnal rhythm of peripheral plasma corticosterone (CORT) and melatonin,and changes of the cholecystokinin (CCK),neuropeptide Y and leptin levels at zeitgeber time (ZT) 0 were detected by using enzyme-linked immunosorbent assay (ELISA).We found that ES led to a disturbance of diumal rhythm of CORT and melatonin in the plasma.Besides,it also increased plasma leptin level and decreased body weight gain,but it did not produce depression-and anxiety-like behaviors compared with those rats in the control group.In short,our findings indicated that the ES could induce a disturbance of diurnal rhythm of plasma CORT and melatonin in male rats.