Tamoxifen is an estrogen receptor antagonist used in the treatment of breast cancer. However, tamoxifen has been shown to induce QT prolongation of the electrocardiogram potentially causing life-threatening polymorphic ventricular arrhythmias. The purpose of the present study was to elucidate the electrophysiological mechanism(s) underlying the arrhythmogenic effects of tamoxifen. We used standard ruptured whole-cell and perforated patch-clamping techniques on rat ventricular myocytes to investigate the effects of tamoxifen on cardiac action potential waveforms and the underlying potassium currents. Tamoxifen (3 µmol/L) markedly prolonged action potential duration, decreased maximal rate of depolarization and decreased resting membrane potential. At this concentration, tamoxifen significantly depressed the Ca²⁺-independent transient outward K⁺ current (I_to), the delayed rectifier K⁺ current (I_sus) and inward rectifier K⁺ current (I_K1) and sodium currents (I_Na in the myocytes. Lower concentrations of tamoxifen (1 µmol/L) also decreased the resting membrane potential and significantly depressed I_K1 to 79±5% (n=5; at -120 mV) of pre-treatment values. The results of this study indicate that inhibition of I_to, I_sus and I_K1 by tamoxifen may underlie action potential prolongation in cardiac myocytes and thereby contribute to prolonged QT interval observed in patients.