The ability of surface isopotential mapping techniques to detect and to localize single and multiple discrete epicardial events was assessed using an isolated, perfused rabbit heart technique. Injury currents were generated by searing the epicardium at two sites separated by approximately 180°, 90° or 45° of rotation in three sets of eight preparations each. Electrocardiographic signals, recorded from 32 electrodes on the surface of the spherical test chamber, were processed by digital computer techniques to construct isopotential maps. Relative dipolarity of the surface pattern was quantitated by a numerical procedure and expressed as the percentage of root-mean-square (RMS) potential attributable to a single dipole source. After placing one burn, surface patterns demonstrated a single potential maximum spatially aligned with the lesion. In the 16 preparations with a second lesion 180° or 90° from the first, two discrete maxima appeared. The maximum related to the second lesion was located overlying the burn; that due to the first remained stationary but decreased in intensity by 67.86 ±23.78 μV (p<0.01). In the eight preparations with the two lesions separated by 45°, only one maximum was observed in a position intermediate between those expected from each burn. Nondipolar residuals were not significantly greater than with one burn (10.66% vs. 10.43%, p>0.1). Thus, surface mapping techniques accurately depict single and dual generator locations unless the two sources are insufficiently separated. Maps depicting the differences in potential before and after the second lesion was induced did, however, demonstrate a maximum spatially aligned with the second burn in all subgroups. The roles of dipole strength, eccentricity and separation in determining surface patterns produced by two simultaneously active dipoles are further explored in an appended numerical simulation. © 1977.