Imaging the myocardial activation sequence is critical for improved diagnosis and treatment of life-threatening cardiac arrhythmias. It is desirable to reveal the underlying cardiac electrical activity throughout the 3-dimensional myocardium (rather than just the endocardial or epicardial surface) from non-invasive body surface potential measurements. A new 3-dimensional Electrocardiographic Imaging Technique (3DEIT) based on the boundary element method (BEM) and multi-objective nonlinear optimization has been applied to reconstruct the cardiac activation sequences from body surface potential maps. Ultrafast CT scanning was performed for subsequent construction of the torso and heart models. Experimental studies were then conducted, during left and right ventricular pacing, in which noninvasive assessment of ventricular activation sequence by means of 3DEIT was performed simultaneously with 3-dimensional intracardiac mapping (up to 200 intramural sites) using specially-designed plunge needle electrodes in closed-chested rabbits. Estimated activation sequences from 3DEIT were in good agreement with those constructed from simultaneously recorded intracardiac electrograms in the same animals. Averaged over 100 paced beats (from a total of 10 pacing sites), total activation times were comparable (53.3±8.1 vs 49.8±5.2 ms), and the relative error between the estimated and measured activation sequences was 0.32±0.06. The present experimental results demonstrate that the 3-dimensional paced ventricular activation sequence can be reconstructed using noninvasive multi-site body surface electrocardiographic measurements and imaging of heart-torso geometry. This new 3-dimensional electrocardiographic imaging modality has the potential to guide catheter-based ablative interventions for the treatment of life threatening cardiac arrhythmias.