Objective: It is unknown whether 4-aminopyridine- (4-AP-) sensitive transient outward K⁺ current (I_to1) and/or Ca²⁺-activated transient outward Cl⁻ current (I_Ca.Cl or I_to2) contribute(s) to phase 1 repolarization of pig ventricular action potential (AP). The purpose of the present study was to determine ionic contribution of the phase 1 repolarization of AP in pig ventricle. Methods: We used whole-cell patch techniques to record APs and membrane currents, and Western immunoblotting analysis to detect expression of I_to1 protein (Kv4.2 or Kv4.3) in pig ventricular myocytes. Results: A transient outward current (I_to) was activated upon depolarization voltage steps to between −10 and +60 mV from −50 mV in pig ventricular cells, and the I_to was resistant to 4-AP application, but sensitive to the inhibition by ryanodine (10 μmol/l) and the Ca²⁺ channel blockade, and the Cl⁻ channel blocker 4,4′-diisothiocyanostilben-2,2′disulfonic acid (DIDS, 150 μmol/l). The current was diminished by external Cl⁻ (Cl⁻_o) replacement and showed a ‘bell-shaped’ I–V relationship at room temperature, typical of I_to2. No difference in I_to2 was observed in the regional cells from epicardium, midmyocardium, and endocardium of left ventricle. APs showed significant phase 1 and ‘spike and dome’ in pig ventricular myocytes. The phase 1 and ‘spike and dome’ of APs were not affected by 4-AP (3 mmol/l), but abolished by replacing Cl⁻_o and by application of 100 μmol/l DIDS, suggesting I_to2 contribution. Western immunoblotting analysis showed no evidence for the expression of 4-AP-sensitive I_to1 channel protein (Kv4.2 or Kv4.3) in pig ventricular cells. Conclusion: The results indicate that 4-AP-sensitive I_to1 is not expressed, and only Ca²⁺-activated I_to2 is present in pig cardiac cells, which contributes importantly to the phase 1 repolarization of ventricular APs in this species.