Early afterdepolarizations (EADs) are one mechanism proposed to cause certain cardiac arrhythmias. We studied the effect of the Ca2+ channel agonist Bay K 8644 (1 x 10(-8) to 5 x 10(-5) M) on normally polarized sheep and canine cardiac Purkinje fiber short segments. EADs occurred with higher Bay K 8644 concentrations and had an average take-off potential of -34 mV. The initiation of EADs was preceded by lengthening of action potential duration and flattening of the plateau. Induction of EADs with Bay K 8644 was enhanced by low stimulation frequencies, lowering of [K]o, addition of tetraethylammonium chloride, or application of depolarizing constant current pulses during the plateau. EADs were abolished by increasing stimulation frequency, raising [K]o, the addition of tetrodotoxin, lidocaine, ethmozin, verapamil, and nitrendipine, or application of repolarizing constant current pulses. Using current pulses to modify the action potential plateau, a steep inverse relationship was found between the EAD peak voltage and its take-off potential, and EADs could be initiated over only a narrow range of take-off potentials. Thus, interventions that suppressed EADs shortened action potential duration or shifted the plateau away from the voltage range needed to initiate EADs. These observations suggest that mechanisms dependent on both time and voltage underlie EADs, and provide a unifying hypothesis for the induction of the EADs. We propose that induction of EADs requires 1) lengthening of action potential duration within a plateau voltage range where 2) recovery from inactivation and reactivation of an inward current possibly carried through Ca2+ channels can occur.