Action potentials were recorded from canine Purkinje fibers to evaluate the effects of external calcium and the slow inward current-blocking agents, manganese and verapamil, on the relationship between action potential duration and diastolic interval during both steady state and premature excitation. Elevated external calcium shortened the action potential without changing the dependence of either steady state or premature responses on the preceding diastolic interval. The effects of elevated calcium resembled those of increased stimulation frequencies in that the duration of the steady state action potential was reduced without a concomitant change in the interval dependence of the premature action potentials. In contrast, manganese and verapamil markedly slowed the recovery of the premature action potential duration with increasing diastolic interval, but did not significantly alter the steady state relationship. On the basis of these experiments, we conclude that (1) the mechanisms underlying the immediate and cumulative cycle length-dependent changes in action potential duration are separate processes; (2) the abrupt decrease in action potential duration following a reduction in cycle length is consistent with the incomplete recovery of the plateau currents, whereas the steady state changes involve a slower process, possibly an increase in potassium conductance mediated by an accumulation of internal calcium; and (3) the slower frequency-dependent changes do not depend on calcium entry through the slow inward current channel.Circ Res 46: 543-552, 1980