Objective To observe the effect of Modified Zhenxuan Granules (MZG) on blood pressure, synthesis and bioavail-ability of nitric oxide (NO), and plasma angiotensin I1 (Ang II) contents of spontaneously hypertensive rats (SHR). Methods Fiftymale SHR were randomly assigned to 5 groups: high-, middle-and low-dose MZG groups(at the doses of 37.50, 18. 70,and 9. 35 g/kg respectively), captopril (25 mg/kg) group and SHR control group. The rats received gastric gavage of corre-sponding medicine according to the experimental design for 12 weeks. Blood pressure from the tail was recorded at the beginning oftreatment and every 4 weeks during 12-weeks treatment. At the end of the experiment, the plasma NO and Ang II concentration,and aortal nitric oxide synthase (NOS) activity and aortal cyclic guanylic acid (cGMP) concentration were measured. ResultsMZG could decrease blood pressure and plasma Ang I1 concentration, increase plasma NO content, and enhance aortal NOS activ-ity and cGMP concentration ofSHR (P <0. 05, compared with the SHR control group), and high-dose MZG had the best effect.After treatment for 12 weeks, high-dose MZG had a similar effect on decreasing blood pressure compared with captoril group,had a better effect on increasing aortal cGMP (P< 0. 05), and increased the bioavailability of NO. Conclusion MZG can de-crease blood pressure, and its mechanism may be related to the improvement of NO/NOS/eGMP system and the decrease of plas-ma Ang Ⅱ concentration in SHR.