Aim: Transgenic rabbits expressing pore mutants of hKCNQ1(KvLQT1-Y315S) display a Long QT-Syndrome 1(LQT1) phenotype. Recently, NS1643 has been described as one of the first drugs to increase IKr/HERG current. We hypothesized that NS1643 would hasten cardiac repolarization and shorten the action potential duration (APD90) in LQT1 rabbits, thereby normalizing the QT interval. Methods: Female transgenic LQT1 rabbits aged 4-6 months were compared to littermate controls (LMC) in vivo and ex vivo. In vivo ECG studies (n=9 vs 6/group) in sedated animals were performed at baseline and during 30min i.v. infusion of NS1643 (1.5mg/kg/min) or vehicle (PEG400/glucose) in a crossover design with 1 week between treatments. Ex vivo 4 left ventricular monophasic action potentials were recorded from Langendorff-perfused hearts (n=13 vs 10/group) at baseline and during 45min perfusion with NS1643 (5mM) at atrial pacing with an RR interval of 400ms. Epicardial left ventricular refractory periods (VERP) were assessed before and after 45min NS1643 infusion. Results: In vivo NS1643 shortened the QTc after 30min significantly in LQT1 by 8.5%±0.7 compared to vehicle 2.5%±1.1(p<0.001), whereas NS1643 did not shorten the QTc significantly in LMC. In Langendorff experiments APD90 was shortened by NS1643 in LQT1 by 32.0ms±4.3 (p<0.01) and in LMC by 21.0ms±5.0 (p<0.01). Concomitantly, the VERP was significantly shortened by NS1643 (LMC:22.6ms±9.9; LQT1:23.7ms±8.3ms). Before NS1643 2 LMC and 5 LQT1 had ventricular fibrillation either spontaneously or during VERP-testing, whereas after NS1643 9/10 LMC and 13/13 LQT1 animals had VF. Additionally, dp/dt was significantly decreased (63%±4 in LMC and 49%±3 in LQT1) after NS1643. Conclusions: We demonstrate that NS1643 shortens the QT intervals in LQT1 significantly in vivo. Ex vivo NS1643 shortens the APD90 and effective refractory period in both genotypes, but is associated with increased arrhythmia susceptibility and negative inotropy.