AIM　To study the positive inotropic effect of methyl polyglycoside (Mpg) on isolated guinea pig atria and its mechanism. METHODS　The effects of Mpg on contractility, beat rate of right atria, post-rest contraction and positive staircase phenomenon were observed in isolated guinea pig left and right atria. Na+-K+-ATPase activity of rat myocardial cell membrane was measured. RESULTS　Mpg (0.01～3 mmol*L-1) produced a concentration-dependent increase in myocardial contractility of atria and decrease in beat rate of right atria. Mpg markedly enhanced the post-rest contraction and the positive staircase phenomenon induced by increasing stimulation frequency in left atria, and inhibited Na+-K+-ATPase activity of rat myocardial cell membrane. CONCLUSION　The results suggest that Mpg can strengthen myocardial contractility of atria and decrease beat rate of right atria. Its positive inotropic effect may be related to inhibiting Na+-K+-ATPase activity of myocardial cell membrane, increasing the transmembrane influx of calcium and the amount of calcium released from intracellular stores.