This study compared the hemodynamic effects of verapamil with other interventions that alter loading conditions in canines. The close correlation between peak filling velocity and systolic excursion indicated that peak filling velocity was dependent on systolic performance. However, verapamil caused a significantly greater increase in peak filling velocity than nitroprusside or volume loading despite a comparable increase in systolic excursion. Although both verapamil and volume loading induced increases in the peak filling velocity, the end-diastolic pressure and the peak pressure of the atrial v-wave, the pressure at the nadir of the atrial y-wave fell after verapamil but rose after volume loading. The rate of y-descent was also significantly greater after verapamil than after volume loading. These results indicate that, in addition to a reduction in afterload and an increase in preload, verapamil has a direct beneficial effect on ventricular filling.