Objective: The rapid (I_Kr) and slow (I_Ks) components of delayed rectifier currents play an important role in determining the cardiac action potential configuration. Abnormalities in their function may contribute to arrhythmogenesis under pathological conditions. We studied the effects of myocardial infarction on I_Kr and I_Ks in canine ventricular myocytes and their molecular basis. Methods: Infarct zone myocytes (IZs) were isolated from a thin layer of surviving epicardium overlying an infarct 5 days after a total occlusion of the left anterior descending (LAD) coronary artery. Normal myocytes (NZs) were isolated from the corresponding region of control hearts for comparison. Currents were recorded under the whole-cell patch clamp conditions. Results: Both I_Kr and I_Ks current densities were reduced in IZs versus NZs. Kinetic analysis further suggests an acceleration of I_Kr activation and I_Ks deactivation. RNase protection assays were used to quantify the mRNA levels of I_Kr and I_Ks channel subunits (dERG, dIsK and dKvLQT1) in tissue immediately adjacent to the region where myocytes were isolated. mRNA levels of all three subunits were reduced 2 days after LAD occlusion (by 48±9%, 68±5%, and 45±4% for dERG, dIsK and dKvLQT1, respectively, n = 8 each). By day 5, the dKvLQT1 message returned to control while those of dERG and dIsK remained reduced (by 52±7% and 76±6%, respectively). Conclusions: The decrease in I_Kr and I_Ks amplitudes and changes in their kinetics in infarcted tissue might be due to a decrease in functional channels and/or changes in their subunit composition. Heterogeneous changes in I_Kr and I_Ks in infarcted hearts may impact on the effects of varying heart rate or neurohumoral modulation on repolarization.