Cesium (Cs) and tetraethylammonium (TEA) have been shown to increase action potential duration. However, action potential duration is known to be influenced by the rate of stimulation. In this study, the effect of stimulation rate on action potential characteristics was studied in Cs-treated and TEA-loaded canine Purkinje fiber preparations. Action potentials of Purkinje fibers from Cs-treated and TEA-loaded preparations had longer durations than action potentials of Purkinje fibers from normal preparations. Greater prolongation of action potential duration was observed when the rate of stimulation was reduced in Purkinje fibers from Cs-treated and TEA-loaded preparations than those from normal preparations. Whereas the increase in action potential duration of Purkinje fibers from Cs-treated preparations was accompanied by a significant membrane depolarization, no change in membrane potential was observed in Purkinje fibers from TEA-loaded preparations. In some Cs-treated and TEA-loaded preparations, the prolonged duration observed at slow stimulation rates was associated with the appearance of early afterdepolarizations. Lidocaine and cromakalim, agents known to reduce action potential duration in normal Purkinje fibers, also shortened action potential duration in Purkinje fibers from both Cs-treated and TEA-loaded preparations. However, lidocaine and cromakalim caused a significant membrane depolarization in Cs-treated Purkinje fibers but not in TEA-loaded Purkinje fibers. Our results suggested that although Cs and TEA are capable of producing rate-dependent prolongation of action potential duration and the occurrence of bradycardia-dependent early afterdepolarization, differences exist in Cs-treated Purkinje fibers in terms of the appearance of membrane depolarization at reduced stimulation rate and in the presence of lidocaine and cromakalim.