Abstract: AimTo determinein vitro the rat plasma protein binding rate by using microdialysis method. MethodsThe binding rate was determined by using microdialysis probe as sampling tools and zero-net flux method as calibrating method. The regression equation was made by the difference of concentrations between the dialysis sample and the perfusate. The x-intercept of regression equation was the free drug concentration (C_f). The plasma protein binding rate was calculated by using the following equation: f=(C₀-C_f)/C₀. ResultThe binding rate was kept relatively stable in the studied concentration range. ConclusionIt is feasible that the plasma protein binding rate can be determined by using microdialysis method.