To study the electrogenesis of the S1S2S3 pattern, seven patients had body surface potential mapping and endocardial mapping of inflow tract, outflow tract, and apex of the right ventricle. QRS duration was longer in S1S2S3 versus controls (94 +/- 14 vs. 84 +/- 14 msec). Surface mapping was similar in S1S2S3 patients and in controls during the first 30-40 msec of QRS, but S1S2S3 patients subsequently presented the following differences: (1) earlier time of onset (34 +/- 3 vs. 44 +/- 6 msec) and a lower voltage (1,242 +/- 468 vs. 1,649 +/- 31 mV) of peak positive anterior maximum; (2) earlier dorsal migration (45 +/- 3 vs. 55 +/- 7 msec) of the maximum; (3) a second peak positive maximum at 58 +/- 3 msec, located on the dorsal spine; (4) the appearance of a right subclavicular positive area at 51 +/- 6 msec, which in controls was absent or appeared later (66 +/- 7 msec). At the end of QRS, the maximum was located in all but two S1S2S3 cases on the upper sternum. Right ventricular endocardial mapping showed a similar activation time of the apex in S1S2S3 patients and controls, but in the former a significant inflow (56 +/- 21 vs. 36 +/- 9 msec) and outflow tract (79 +/- 13 vs. 39 +/- 8 msec) activation delay was documented. The data obtained using body surface potential mapping suggest that an anomalous wavefront rightward and superiorly oriented is present in the S1S2S3 pattern, which is able to oppose the electrical forces of ventricular free walls.(ABSTRACT TRUNCATED AT 250 WORDS)