The time of the minimum slope (i.e., the fastest negative deflection) in monopolar (MP) electrograms from normal hearts compares closely with time of phase 0 of the action potential in cells underlying the electrode, but poor rejection of far-field activity may limit the utility of MP electrode technology in dense arrays used for the study of ventricular tachycardia and fibrillation. The purpose of this study is to evaluate more myopic discrete bipolar (BP) and nondirectional, two-dimensional current source density (CSD) based arrays for rejection of far-field potentials and precision of activation time determination. Simultaneous recordings of the CSD, MP, and multiple BP electrograms were performed on normal dog epicardium. The time of the minimum slope in MP electrograms was compared to activation times in CSD and BP derivations using: (1) peak; (2) steepest slope; (3) zero crossing of the steepest sloping segment in either direction; and (4) waveform morphology. In vivo, CSD amplitude was reduced significantly more than MP and BP amplitudes by insertion of inert media between the heart and the electrodes. The time of the steepest slope in CSD electrograms designated activation times closest to the time of the minimum slope in MP electrograms (0.9 +/- 1.3 msec). We conclude that CSD provides a nondirectional electrode system that accurately defines the time of local activation and possesses better spatial specificity than MP electrode systems and BP electrode systems having the same interelectrode distances.