The objective of the present work is to study the ionic mechanisms for the regional differences in action potential duration in rat ventricle. This regional diversity has been related to differences in the regional distribution of some potassium currents in several species. Single cells were obtained by enzymatic dispersion of tissue segments from rat ventricular muscle. Whole cell voltage-clamp methods were used to identify the K+ currents involved in action potential repolarisation in the different regions. 4-Aminopiridine, TEA and voltage protocols were used to isolate the following potassium currents: transient outward, Ito, delayed rectifier, Ik, and sustained current, Iss. In the present work, we have studied the distribution of these three repolarising currents, and that of the inward rectifier, Ikl, in the free wall of the right ventricle, the subepicardium of the apex of the left ventricle and in the subendocardium of the base of the left ventricle. Action potential duration was longer in the left than in the right ventricle, and in the former it was longer in the subendocardium of the base than in the subepicardium of the apex. The main difference was in the phase 1, suggesting the implication of Ito. This was confirmed with voltage-clamp experiments. In conclusion, this work shows that Ito current density is higher in the regions with the shorter action potential, whereas there are no differences in the regional distribution of Ik, Iss or Ikl.