Abstract: The effects of methylflavonolamine hydrochloride (MFA) on the contractility, excitability, automaticity and functional refractory period (FRP) were studied in isolated guinea pig atrium. MFA (5 and 75 μmol/L) decreased the contractility, increased the threshold concentration of adrenaline to evoke automaticity of the left atria, depressed the spontaneously beating right atria and lengthened FRP. MFA, at 75μmol / L but not at 5/μmol/L, and shifted the intensity duration curve to the right. Our results suggest that supression of the spontaneous and the evoked automaticity and prolongation of FRP may be related to its antiarrhythmic action.