Background — We hypothesize that inferior vena cava-inferior atrial ganglionated plexus nerve activity (IVC-IAGPNA) is responsible for the ventricular rate (VR) control during atrial fibrillation (AF) in ambulatory dogs. Methods and Results — We recorded bilateral cervical vagal nerve activity (VNA) and IVC-IAGPNA during baseline sinus rhythm and during pacing-induced sustained AF in 6 ambulatory dogs. Integrated nerve activities and average VR were measured every 10-s over 24-hour periods. LVNA was associated with VR reduction during AF in 5 dogs (from 211짹28 bpm to 178짹38 bpm, p<0.001) and RVNA in 1 dog (from 208짹21 bpm to 181짹23 bpm, p<0.01). There were good correlations between IVC-IAGPNA and LVNA in the former 5 dogs (average r of 0.792짹0.103, p<0.05 in all), and between IVC-IAGPNA and RVNA in the latter dog (r=0.773, p<0.05). IVC-IAGPNA was associated with VR reduction in all dogs studied (r=-0.404짹0.102, p<0.05). Average VR during AF was 184짹45 bpm (67 bpm – 269 bpm). The average VR during IVC-IAGP discharges was 168짹46 bpm, while the average VR without IVC-IAGP discharges was 194짹46 bpm (p<0.001). RVNA was associated with baseline sinus rate reduction (from 105짹21 bpm to 77짹15 bpm, p<0.01) in 4 dogs and LVNA in 2 dogs (from 111짹18 bpm to 81짹18 bpm, p<0.01). Conclusions — IVC-IAGPNA is invariably associated with VR reduction during AF. On the other hand, right or left VNA was associated with VR reduction only when it co-activates with the IVC-IAGPNA. The VNA that controls VR during AF may be different than the VNA that controls baseline sinus rhythm.