Objective:To sutdy the enzyme-reducing action and pharmacodynamics of Xiaoyao powder on liver injury in rats in the morning and evening in order to provide chronomedication evidence for hepatopaths.Method:The liver injury rat model was developed by subcutaneous injection of forty percent carbon tetrachloride every two days.With the decrease of aspartate amino transferase as the parameter

the pharmacodynamics in the morning and evening of Xiaoyao powder was studied.The apparent parameters of pharmacodynamics were estimated based on the time-effect curve by PKSolver 2.0.Result: The time-effect curve of Xiaoyao powder shows characteristics of double peaks and can be described by statistical moment.The main pharmacodynamic parameters are as follows: td = 17.79 h（day）

24.30 h（night）

t1/2（ED） = 3.73 h（day）

3.50 h（night）

AUC = 657.07 U.L-1.h-1（day）

810.19 U.L-1.h-1（night）

MRT = 6.23 h（day）

6.19 h（night）

respectively.Conclusion: Significant circadian rhythm of pharmacodynamics was found in rats when Xiaoyao powder was given at different times of the day.The findings suggest that the hepatic protection effect for liver injury of Xiaoyao powder in rats is better when given at the end of rest phase than other time session.