<Background> It is not known that an S3 can be audible intermittently. We have recognized this in patients with VVI pacing in whom sinus rhythm has been preserved. <Subjects and methods> Subjects consisted of consecutive 39 patients with VVI pacemaker implantation and preservation of sinus rhythm. We tried to find out what percentage of these patients have an intermittent S3 and also to elucidate the mechanism of this sound by Doppler echocardiography. The PP interval and RR intervals were measured and ｜PP-RR｜/RR was calculated. One doctor carried out auscultation from the left sternal border to the apex for 5-6 consecutive beats while the patients held their breath. Transmitral flow velocity was measured by pulsed Doppler echocardiography with a paper speed of 100 cm/sec. A wave velocity was measured when a P wave coincided with late diastole. E wave velocity was measured when no A wave was present in early diastole. The maximal summation of E and A waves (E+A) velocity was measured. <Result> An intermittent S3, being audible in 21 patients (group A) and not audible in 18 patients (group B), coincided with maximal E+A velocity. There was no statistical age difference between the two groups. Both maximal E+A velocity, and A velocity were higher in group A than group B (116.2 cm/sec Vs 90.8 cm/sec P<0.0001, 80.6 cm/sec Vs 56.4 cm/sec P<0.0001 respectively), while E velocity was not statistically different (68.3 cm/sec Vs 60.3 cm/sec). In 5 of 9 cases in whom an intermittent S3 was not audible regardless of more than 100 cm/sec of E+A velocity, the calculated ｜PP-RR｜/RR was less than 0.1. <Conclusion> An intermittently audible S3 is one of the physical signs of complete atrio-ventricular block. It occurs when a strong atrial contraction develops exactly at the time of rapid left ventricular filling. However, when the PP and RR intervals are too close, the S3 may be difficult to discern.