Aim: To investigate the possible synergism of hydrochlorothiazide and nitrendipine on reducing both blood pressure (BP) and blood pressure variability (BPV) in spontaneously hypertensive rats (SHR). Methods: Seventy animals were randomly divided into seven groups. The doses were 5 and 10 nig/kg for nitrendipine, 10 and 20 mg/kg for hydrochlorothiazide and 10+5, 20+10 mg/kg, respectively, for the combination of these two drugs and 0.8% carboxym-ethylcellulose as control. The drugs were given via a catheter of gastric fistula. BP was then continuously recorded for 5 h from 1 h before drug administration to the end of 4th hour after drug administration, in conscious and freely moving rats. Results: The effects on both BP and BPV reduction of the combination of hydrochlorothiazide and nitrendipine were greater than the single drug in SHR. The two drugs possessed an obvious synergism on both systolic blood pressure (q=1.79 with small dose and q=1.23 with large dose) and systolic blood pressure variability reduction (q=1.79 with small dose and q=1.39 with large dose) in SHR.Conclusion: The present work clearly demonstrated that there was a synergistic effect between hydrochlorothiazide and nitrendipine in lowering and stabilizing BP in SHR.