The effects of verapamil administration or vagal stimulation on the mechanical restitution curve (MRC) were studied in order to better understand the modulation of left ventricular (LV) function by interventions that lower the ventricular rate of atrial fibrillation. The MRC and the postextrasystolic MRC were obtained in 11 dogs using peak single beat elastance (Emax). The MRC was fitted by a monoexponential curve. Vagal stimulation or verapamil administration decreased the peak of the MRC and right-shifted the MRC. The postextrasystolic MRC was located upward compared with the control MRC, and was shifted downward by vagal stimulation or verapamil administration. If interventions having a negative inotropic effect effectively slow a rapid heart rate, the net effect of the ventricular contractile state may not always be negative. It was concluded that the MRC is useful in understanding LV contractility during irregular rhythm, especially when assessing the net effect of the negative dromotropic and inotropic action of antiarrhythmic drugs.