Seven cases of ventricular cross stimulation from a group of 23 patients implanted with DDD devices are presented. In two patients the phenomenon was observed at the moment of DDD programming at nominal values, and in five other patients it was reproduced by increasing the atrial output voltage up to ten volts. In all 23 patients cross stimulation disappeared permanently within 24 hours after implantation. From the onset of cross stimulation to its end, atrial and ventricular threshold voltages were unchanged, while the atrial and ventricular impedances significantly decreased. These results suggest that an important role in the phenomenon occurs by impedance variation at the interface between the pulse generator and body tissue.