Backgrounds: The reentry circuit of typical slow/fast atrioventricular nodal reentrant tachycardia (AVNRT) has not been delineated completely. It has been proposed that the left atrial septum and proximal coronary sinus (CS) musculature may participate in the reentry circuit of AVNRT. We performed electrophysiological study to investigate whether CS musculature is included in the reentry circuit of AVNRT. Methods: A total of 13 patients with typical slow/fast AVNRT were included. Single extrastimuli were delivered at the posteroseptal, midseptal regions and inside the CS (5-10 mm from the CS ostium) during tachycardia, beginning with the tachycardia cycle length (CL) and decreasing by 10ms until the next His potential was advanced. The anatomical location of CS ostium was confirmed by CS angiography in each case. The longest coupling intervals (CI) and return cycles (RC) of single extrastimuli that reset the tachycardia were evaluated. Results: 1. Tachycardia cycle length was 348짹50ms. The earliest atrial activation was recorded at the His bundle region, and the earliest CS activation was recorded at the CS ostium in all patients. 2. The longest CIs of single extrastimuli that reset the tachycardia were 293 짹49ms inside the CS, 331짹87ms at midseptal and 332짹67ms at posteroseptal area. At these CIs, RCs were 401짹64ms inside the CS, 373짹95ms at midseptal and 373짹85ms at posteroseptal area. 3. Successful slow pathway elimination or modification was achieved in all patients by radiofrequency energy application at the posteroseptal or midseptal regions. Conclusion: The reentry circuit of typical slow/fast AVNRT includes the midseptal and posteroseptal atrioventricular junctional regions while the proximal CS musculature may not be part of the reentry circuit in typical slow/fast AVNRT.