Nine patients of mean age 47.8 years, with suspected sinus node dysfunction, underwent extensive electrophysiological studies. Sinus bradycardia(6the electrophysiological study, AH and HV intervals were prolonged in 2/9 and 1/9 patients, respectively. Maximal sinus node recovery times were prolonged in 7/9 patients, ranging from 1,330 msec to 12,330msec. Sinoatrial conduction times measured by atrial premature stimulation technique were prolonged in 5/7 patients, ranging from 137msec to 310 msec. And sinoatrial conduction time measured by continuous pacing technique also revealed prolonged value in 4/6 patients ranging from 140 msec to 195 msec. The effective refractory periods, of atrium were prolonged in 6/8 patients (320 msec to 470 msec). The effective and functional refractory periods of AV node were prolonged in 3/8 patients (440 csec to 490 msec) and 4/8 patients (530 msec to 560 msec), respectively. Retrograde VA conduction could be observed in 3/7 patients and ventricular effective refractory periods were normal in 7/7 patients. Atrial flutter was induced in 1/9 patients by electric stimulation during electrophysiological study.