Background: A single stationary mother rotor has been hypothesized to be responsible for maintenance of ventricular fibrillation (VF) in the guinea pig. Previous studies have pointed to the ventricular septum as a possible location for a mother rotor in the pig heart. Objectives: The purpose of this study was to test the hypothesis that a mother rotor is located in the septum. Methods: In seven open-chest pigs, we mapped the first 20 seconds of electrically induced VF simultaneously from the posterior left ventricle (LV) and right side of the septum with two electrical arrays. Each array contained 504 electrodes (21 × 24) spaced 2 mm apart in the LV and 1.5 mm apart in the septum. Results: The percentage of VF wavefronts that formed reentrant circuits was significantly lower in the septum (1% ± 1% [mean ± SD]) than in the LV (2% ± 1%). The peak frequency during VF also was significantly smaller in the septum (8.6 Hz ± 3.0 Hz) than in the LV (10.4 Hz ± 3.4 Hz). The mean direction of spread of activation of VF wavefronts was away from the region where the posterior LV free wall intersects the posterior septum in both the LV and septum. Conclusions: The lower incidence of reentry and lower peak frequency in the mapped region of the septum than in the LV indicate that a mother rotor is not present in swine on the RV side of the septum. The mean directions of the VF activation sequences in the LV and septum suggest that if a mother rotor is present during the first 20 seconds of VF, it exists where the posterior LV free wall joins the septum, the region where the posterior papillary muscle inserts. © 2005 Heart Rhythm Society. All rights reserved.