Long-QT syndrome is characterized by prolongation of the electrocardiographic QT interval and is associated with syncope and sudden cardiac death. One implicated mutation (SCN5ADKPQ) is predicted to result from delayed inactivation of the cardiac sodium channel although downstream functional consequences are not understood. We have characterized the electrophysiological phenotype of ventricular myocytes from mice harbouring this mutation. Enzymatically dissociated adult ventricular myocytes were prepared from wild-type and SCN5ADKPQ mice as described by Ashely (2002). Hearts were excised from 20-25 g adult C57BL mice after schedule 1 humane killing by cervical dislocation. SCN5A-DKPQ showed a significant prolongation of action potential (AP) duration (152 ± 18 ms, n=9) compared to wild-type (55 ± 6.6 ms, n=9, p<0.01) and earlyafterdepolarization like AP. The current density of iNa and late current (see Fig. 1) in SCN5A-DKPQ myocytes are also increased. These findings suggest that the late current could be a major mechanism responsible for the ventricular arrhythmias associated with this mutation.