Monensin, a monovalent cation ionophore, transports sodium ions preferentially. We found that, in the cardiac Purkinje fibers, monensin 10(-5) M increased the resting tension of the fiber bundle in Tyrode solution containing 4.5 mM Ca. This ionophore (10(-5) M) shortened the duration of the action potential and suppressed the pacemaker potential. In Na-free or Ca-free solutions, monensin had no effect on the configuration of the action potential. In voltage clamp experiments, monensin 10(-5) M shifted the holding current at -40 mV outwardly, increased the instantaneous inward current (possibly inward rectifying potassium current, IK1) and increased the transient outward current (Ito), whereas it attenuated the hyperpolarization-activated inward current (If). The delayed rectifying outward current (IK) was not significantly affected by monensin 10(-5) M. The transient inward current (ITI) appeared in the presence of monensin 10(-5) M. These changes induced by monensin are consistent with changes in configuration of the action potential induced by monensin. The membrane current changes are considered to be induced by an increase in intracellular Ca concentration, probably via a Na-Ca exchange following an increase in intracellular Na concentration, and by alteration of intra- and extracellular Na and K concentrations.