We evaluated the ability of a computer procedure to locate cardiac electrical activity from body surface potentials during normal and ectopic beats in six intact dogs. The location of the dipole that best reconstructed the signals recorded from 26 thoracic electrodes was computed by using a torso model. This model was designed from geometrical measurements made on the first dog. Chronically implanted subepicardial electrodes produced ectopic foci at three known locations: apex, and right and left ventricles. For the normal QRS complex, the dipole started in the middle of the septum, moved upward, then to the left, downward, and back to the right; it remained stationary at the level of the base during repolarization. After ectopic stimulation the dipole started its course at a distance of 1.9 ± 0.8 (SD) cm from the stimulus site and then traversed the heart, moving away from the ectopic site; during the T wave, it roughly followed the QRS path, but at a slower speed. The speed of the dipole ranged between 0 and 6 m/sec, and its orientation often did not coincide with the direction of its path. The location of the dipole appeared to be close to the corresponding wavefronts when these were unique and dipolar, especially during the early and terminal portions of the QRS. The results show that the initial location of the dipole can give the approximate position of an ectopic focus in vivo, and that the trajectory of the dipole during QRS can portray the passage of an ectopic beat across the heart. Circ Res 46: 415-425, 1980