Piperlonguminine,the effective composition of the Mongolian medicine Piper longum L.,has fluorescence and its fluorescence signal could be greatly enhanced by the surfactant Tween -20.The synchronous fluorescence spectroscopic method could eliminate the interference of the background fluorescence from serum samples on fluorescence spectra of piperlonguminine.According to this technique,a method was established for the determination of piperlonguminine in rat serum.The linear range of this method was in the range of 5.47×10-3-3.28 μg?mL-1,the detection limit for piperlonguminine was 1.20×10-3 μg?mL-1,and the relative standard deviation(RSD) was 2.7%.The method was successfully applied in the detection of piperlonguminine in rat serum amples,with recovery of 100.6%.