T wave alternans, characterized by a beat-to-beat change in T wave morphology, amplitude, and/or polarity on the ECG, often heralds the development of lethal ventricular arrhythmias in patients with left ventricular hypertrophy (LVH). The aim of our study was to examine the ionic basis for a beat-to-beat change in ventricular repolarization in the setting of LVH. Transmembrane action potentials (APs) from epicardium, endocardium were recorded simultaneously, together with transmural ECG and contraction force, in arterially perfused rabbit left ventricular wedge preparation. APs and calcium-activated chloride current (I_Cl(Ca)) were recorded from left ventricular myocytes isolated from normal rabbits and those with renovascular LVH using the standard microelectrode technique and a whole-cell patch-clamping techniques, respectively. In the LVH rabbits, a significant beat-to-beat change in endocardial APD created beat-to-beat alteration in transmural voltage gradient that manifested as T wave alternans on the ECG. Interestingly, contraction force alternated in an opposite phase (" out of phase")with APD. In the single myocytes of LVH rabbits, a significant beat-to-beat change in action potential duration (APD) was also observed in both left ventricular endocardial and epicardial myocytes at various pacing rates. APD alternans was suppressed by adding 1 µM ryanodine, 100 µM 4,4’-diisothiocyanostilben-2, 2’disulfonic acid (DIDS) and 100 µM 4-acetamido-4’-isothiocyanostilbene-2,2’-disulfonic acid ( SITS). The density of the calcium-activated chloride currents (I_Cl(Ca)) in left ventricular myocytes was significantly greater in the LVH rabbits than the normal group. Our data indicate that abnormal intracellular calcium fluctuation may exert a strong feedback on the membrane I_Cl(Ca) current, leading to a beat-to-beat change in the net repolarizing current that manifest as T wave alternans on the ECG.