Objective:To observe the tolerance and the dependence of endomorphin-1(EM-1) in rats and the possible mechanisms. Methods:Sixty Sprague-Dawley rats were randomly allocated into saline, acute EM-1-treated and chronic EM-1-treated groups. The rats were intracerebroventricularly injected with saline, acute EM-1 10 -滋g/kg 30 min prior to sacrifice,and chronic EM-1 by daily administration at 8:00 A.M. and 15:00 P.M. from 10 -滋g/kg on the 1st day to 50 -滋g/kg on the 9th day, respectively. In chronic EM-1-treated group, the median antinociceptive dose(AD50) and the catatonic median effective dose(ED50) were determined by the improved Dixon＇s method. Natural withdrawl test was used to assess the dependence of EM-1. Maximal binding capacity(Bmax) and dissociation constant(Kd) of 3H-DAMGO, binding to mu-opioid receptor(MOR) in brain tissue, was measured by Scatchard analysis. Gene expression of MOR was measured by reverse transcription-polymerase chain reaction(RT-PCR). Results:Tolerance of the antinociceptic and catatonic effects on the 3rd day(3.1-fold and 1.9-fold) and the 9th day(28.4-fold and 8.5-fold) were observed in chronic EM-1-treated group(P < 0.05). Jumping times and withdrawal scores of rats were significantly higher in the chronic EM-1-treated group than those in saline group on the 9th day(P < 0.05). Bmax and mRNA expression of MOR in cortex, midbrain and striatum were lower in chronic EM-1-treated group on the 9th day than the other two groups(P < 0.05), but Kd had no significant difference(P > 0.05). AD50,ED50,Bmax,Kd and gene expression of MOR were recorded. Conclusion: EM-1 possesses the tolerance and the dependence. After a long-term treatment, EM-1 down regulates the binding capacity and mRNA of MOR, which somewhat accounts for the dependence.