Hypertrophy and heart failure are associated with an enhanced propensity for cardiac arrhythmias and a high mortality rate. Altered repolarization might play a role in the occurrence of these ventricular arrhythmias. Beat-to-beat variability of repolarization duration (BVR) has been proposed as a parameter for detection of an unstable, and less controlled repolarization process that precedes the actual tachyarrhythmia. To investigate the relevance of BVR in identifying individuals at risk for arrhythmic events, this parameter was studied in dogs with remodeled hearts and increased susceptibility to arrhythmias due to chronic complete atrioventricular block. Progression of electrical remodeling (prolongation of repolarization times), vulnerability to arrhythmias and sudden cardiac death were reflected in baseline values of BVR. Furthermore, BVR showed a strong predictive value in the screening for pro-arrhythmic effects of drugs. Thus, BVR can be used to identify 1) individuals at risk for ventricular tachycardias and 2) drugs with proarrhythmic properties.