Background —The ventricular action potential exhibits regional heterogeneity in configuration and duration (APD). Across the left ventricular (LV) free wall, this is explained by differences in repolarizing K ⁺ currents. However, the ionic basis of electrical nonuniformity in the right ventricle (RV) versus the LV is poorly investigated. We examined transient outward ( I _TO1 ), delayed ( I _Ks and I _Kr ), and inward rectifier K ⁺ currents ( I _K1 ) in relation to action potential characteristics of RV and LV midmyocardial (M) cells of the same adult canine hearts. Methods and Results —Single RV and LV M cells were used for microelectrode recordings and whole-cell voltage clamping. Action potentials showed deeper notches, shorter APDs at 50% and 95% of repolarization, and less prolongation on slowing of the pacing rate in RV than LV. I _TO1 density was significantly larger in RV than LV, whereas steady-state inactivation and rate of recovery were similar. I _Ks tail currents, measured at −25 mV and insensitive to almokalant (2 μmol/L), were considerably larger in RV than LV. I _Kr , measured as almokalant-sensitive tail currents at −50 mV, and I _K1 were not different in the 2 ventricles. Conclusions —Differences in K ⁺ currents may well explain the interventricular heterogeneity of action potentials in M layers of the canine heart. These results contribute to a further phenotyping of the ventricular action potential under physiological conditions.