Objective The aim of this study was to establish a rat model of Parkinson's disease (PD) by using 6-hydroxydopamine (6-OHDA) and detect the salsolinol N-methyltransferase (SNMT) activity in peripheral lymphocytes of PD rats for the development of a biomarker for early diagnosis of PD. Methods Rat model of PD was established by unilateral double-pointed injection of 6-OHDA into the striatum and was verified by behavior observation. An analytical method was developed based on multiple reaction monitoring with HPLC-ESI-QQQ to determine the SNMT activity in peripheral lymphocytes. Results Seven of 18 rats injected with 6-OHDA showed steadily apomorphine-induced rotation (>7 r/min). The success rate was 38.9%. A sensitive and stable quantitative method with internal standard added was created, based on multiple reaction monitoring mode to analyze SNMT activity. The limit of detection (LOD) and limit of quantitation (LQD) of N-methyl-salsolinol, which is the product of Salsolinol catalyzed by SNMT, were 49 pmol/L and 98 pmol/L, respectively. The precisions of intra-day and inter-day assays both were below 6.0%. SNMT activity of peripheral lymphocytes in the 6-OHDA-lesioned rats was significantly increased[43.37±9.49 pmol/(h·mg)NMSal] in comparison with that in the normal group[2.16±5.82 pmol/(h·mg)NMSal] and the sham-operated group[0.58±2.32 pmol/(h·mg)NMSal](P< 0.01, n=5). There was no significant difference between the normal group and sham-operated group (P< 0.05, n=5). Conclusions Our results indicate that SNMT activity may reflect the changes in the course of Parkison's disease and may become a potential clinical biomarker in diagnosis of this disease.