Aim To investigate the role of melatonin in the improvement of blood pressure and mesenteric arterial function induced by aerobic exercise in spontaneously hypertensive rat （SHR）. Methods A total of 72 twelve-week-old male rats were used in the experiments. A group of normotensive WKY rats （n＝18） was used as normal blood pressure control group （WKY group）. 54 SHRs were separated into five groups: SHR sedentary control group （SHR-SED, n＝18）, SHR sedentary with normal saline injection group （SHR-SED＋NS, n＝6）, SHR sedentary with melatonin receptor blocker luzindole （Luz） injection group （SHR-SED＋Luz, n＝6）, SHR aerobic exercise group （SHR-EX, n＝18）, and SHR aerobic exercise with Luz injection group （SHR-EX＋Luz, n＝6）. Luz was injected 1 mg/（kg·d） intraperitoneally. Rats in the aerobic exercise groups were subjected with 8 weeks-treadmill exercise: 0° slope, 18～20 m/min, 60 min each day, 5 days each week. Blood pressure and heart rate （HR） of awake animals were measured by using tail-cuff method before the beginning of the exercise training protocol and at the end of the protocol. Serum melatonin level was examined by ELISA with commercially available kits. The mechanical properties of small mesenteric arteries were studied with vascular ring experiment in vitro. Results （1）After exercise training for 8 weeks, systolic blood pressure （SBP）, diastolic blood pressure, mean arterial pressure and HR were significantly lower in SHR-EX group than those in SHR-SED group. SBP and HR in SHR-EX＋Luz group were greater compared to SHR-EX group. However, there were no significant differences of SBP and HR between SHR-SED group and SHR-SED＋NS group, SHR-SED＋Luz group. （2）The serum melatonin levels in WKY group and SHR-SED group reached peak level at 21∶00～22∶00, and were significantly higher than those at 12∶00～13∶00 and 8∶00～9∶00. However, it reached the peak level at 8∶00～9∶00 in SHR-EX group. In these three time periods of day, serum melatonin concentration in WKY group was always significantly higher than that in SHR-SED group But it was significantly higher at 8∶00～9∶00 in SHR-EX group than those in WKY group and SHR-SED group. （3）Melatonin （10^－6～10^－3 mol/L） caused a concentration-dependent relaxation in small mesenteric arterial beds precontracted by norepinephrine （10^－5 mol/L）, with pIC50 value∶ WKY group>SHR-EX group>SHR-SED group. In the presence of L-nitro-arginine-methyl ester （L-NAME） （eNOS inhibitor）, the pIC50 of the concentration-response curve for melatonin-induced relaxation was lower than that in the absence of L-NAME in both SHR and WKY rats. The relaxation of melatonin was partly blocked by Luz （2×10^－6 mol/L）. Conclusion Aerobic exercise can reduce SBP and HR of SHR, and enhance mesenteric artery vasodilation, in which melatonin plays an important role.