Introduction: The His-Purkinje system has been shown to harbor triggers for ventricular fibrillation (VF) initiation. However, the substrate responsible for VF maintenance remains elusive. We hypothesized that standard, electrode-based, point-to-point mapping would yield meaningful insight into site-specific patterns and organization which may shed light on the critical substrate for maintenance of VF. Methods: VF was induced under general anesthesia by direct current (DC) application to the right ventricle in 7 acute canines. A standard EPT Blazer mapping catheter (Boston Scientific, Natuck, MA) was used for mapping in conjunction with a Prucka recording system. We collected 30 consecutive electrograms at 24 distinct sites, confirmed by fluoroscopy and intracardiac echo. These sites included both endocardial and epicardial locations throughout the ventricles and conduction system. Results: A total of 5040 individual data points were collected in 7 separate canine studies. During VF mapping, a transmural disparity was found between the epicardium (average cycle length [CL] of 1136 m s) and the endocardium (average CL of 123 m s) with a p value of