In this study simultaneous measurements of action potentials and force of contraction of isolated ventricular muscle from cat and calf hearts were performed under stimulated steady-state and rested-state conditions. It is shown that time-to-peak force of the rested-state contraction correlates well with electrically induced changes in the duration of the platcau phase of the corresponding action potential. Moreover, adrenaline (0.2 and 0.6 μM) and theophylline (2 and 5 mM) increase the amplitude of the rested-state and stimulated steady-state contractions. The concentrations of these drugs were about equi-effective in increasing stimulated steady-state force of contraction. However, the effect of adrenaline on the rested-state contraction is small in comparison to that of theophylline. The effect of both drugs on the rested-state contraction is accompanied by an increase in the plateau amplitude of the corresponding action potential. Since the plateau phase of the cardiac action potential is maintained by the slow inward current mainly carried by Ca ions and since adrenaline and methylxanthines increase this current, it is concluded from the present study that the rested-state contraction may be regulated primarily by Ca ions flowing into the cell during the action potential, while the stimulated steady-state contraction seems to be determined by the amount of Ca ions released from the sarcoplasmic reticulum.