Objective To study the effect of recombinant human relaxin (RLX) on regulating the protein kinase G (PKG) in myocardial tissue taken from a rabbit diastolic heart failure (DHF) model.Methods A DHF model of rabbits was established by constricting their abominal aorta.Twenty-eight New Zealand rabbits were randomly divided into sham operation group (n =6),DHF group (n=6),30 μg/kg · d RLX group (n=8),98 μg/kg · d RLX group (n=8).The animals were treated with RLX for 2 weeeks.Serum samples were taken at week 10 after operation for measuring the serum levels of BNP,RLX,3-NT,NO,cGMP and PKG by ELISA.Results The serum levels of BNP and 3-NT were significantly higher while those of NO,cGMP and PKG were significantly lower in DHF group,30 μg/kg · d RLX group and 98 μg/kg · d RLX group than in sham operaion group (P＜0.05).The serum levels of NO,cGMP and PKG were significantly higher while those of 3-NT were significantly lower in 98 μg/kg · d RLX group than in DHF group (P＜0.05).Conclusion Large RLX dose alleviates the left ventricular diastolic function and oxidative stress,increases the bioavailability of NO and the activity of PKG through the signal pathaway of NO,cGMP,PKG,and can thus prevent myocardial fibrosis and improve the left ventricular diastolic function in DHF rabbits.