A new mouse cardiac electrophysiology method was used to study mice harboring an alpha-myosin heavy chain Arg403Gln missense mutation (alpha-MHC403/+), which results in histological and hemodynamic abnormalities characteristic of familial hypertrophic cardiomyopathy (FHC) and sudden death of uncertain etiology during exercise. Wild-type animals had completely normal cardiac electrophysiology. In contrast, FHC mice demonstrated (a) electrocardiographic abnormalities including prolonged repolarization intervals and rightward axis; (b) electrophysiological abnormalities including heterogeneous ventricular conduction properties and prolonged sinus node recovery time; and (c) inducible ventricular ectopy. These data identify distinct electrophysiologic abnormalities in FHC mice with a specific alpha-myosin mutation, and also validate a novel method to explore in vivo the relationship between specific genotypes and their electrophysiologic phenotypes.