Transgenic expression of SCN5A mutation N1325S creates a mouse model for type-3 long QT syndrome (LQT3), TG-NS/LQT3. Optical mapping is a high temporal and spatial resolution fluorescence mapping system that records 256 action potentials simultaneously in a Langendorff-perfused heart. Here for the first-time, we provide a spatial view of VT in a genetic LQT3 model using optical mapping. Spontaneous VT was detected in TG-NS/LQT3 hearts, but not in littermate control hearts. VT was initiated primarily by activation of a new firing focus as well as functional conduction block of new activation waves. New firing was initiated at many different Loci in the heart, suggesting that "increased automaticity" is a key mechanism for initiation of VT. The sustained VT was maintained by a reentry mechanism. Nifedipine, an L-type calcium channel blocker, decreased the frequency of VT, indicating the involvement of abnormalities of the calcium homeostasis in the genesis of VT in TG-NS/LQT3 mice. (c) 2006 Elsevier Inc. All rights reserved.