The aim of this study was to evaluate the importance of a normal ventricular activation pattern for cardiac performance. In nine mongrel dogs, atrial pacing was compared to AV synchronous pacing at three different AV delays (150, 100, and 60 ms). In six dogs, proximal septal AV synchronous pacing was compared to apical AV synchronous pacing at three different AV delays. AV synchronous pacing was performed after RF induced complete heart block. Hemodynamics were evaluated by assessment of positive and negative dP/dt, cardiac output, and left ventricular and pulmonary pressures. Atrial pacing was superior to AV synchronous pacing with respect to positive and negative dP/dt and cardiac output. This difference was present at all AV delays. Proximal septal pacing was associated with a higher positive and negative dP/dt compared to apical pacing at all AV delays. Left ventricular activation time was significantly shorter during proximal septal pacing than during apical pacing (88 +/- 4 vs 115 +/- 4 ms, P < 0.001). We conclude that atrial and proximal septal pacing improves cardiac function and shortens the ventricular activation time compared to apical AV synchronous pacing independent of the AV interval.