Electrophysiological studies were performed to see the effects of lidocaine on the conduction system, particularly sinus node and atrium in 40 patients of SSS, using HBE recordings, rapid atrial pacing and atrial extrastimulus technique. Sinus cycle length, PA (P'A), AH, HV intervals, calculated SACT, and refractory periods of atrium, AV node and His-Purkinje system did not change after lidocaine. Only maximum CSRT was significantly increased with lidocaine. These results were not affected by pretreatment of atropine. In conclusion, the combining rapid atrial pacing with lidocaine may be useful to manifest the masked sinus node abnormalities. It was suggested that lidocaine directly depressed sinus node automaticity in SSS patients, without affecting perinodal tissue. Therefore, lidocaine should be used with caution in patients with known or suspected SSS.