In order to determine the significance of prolongation of the direct sinoatrial conduction time (DSACT), an attempt was made to record the sinus node potential in 110 patients which was successful in 84 cases. The DSACT was normal in 45 cases (Group A) and prolonged (over 130 ms) in 39 cases (Group B). The symptomatology, standard ECG and the results of investigation of sinus node function by atrial stimulation of the two groups were compared. The DSACT was prolonged in all 13 patients with paroxysmal sinoatrial block or the sick sinus syndrome, in 71% of 15 patients with permanent sinus bradycardia, in 88% of 22 patients with a corrected sinus node recovery time of over 525 ms, in 82% of 38 patients with a sinoatrial conduction time estimated by the extrastimulus method of over 130 ms or an abnormal zone II, in 80% of 39 patients with sinoatrial conduction times estimated by Narula's method of over 130 ms; therefore, 87% of the 35 patients with probable sinus node dysfunction had long DSACT. On the other hand only 2 out of 35 patients (6%) with apparently normal sinus node function had prolonged DSACT. These results indicate that prolongation of the DSACT is a sensitive and specific criterion of sinus node dysfunction. In cases of sinus node dysfunction dizziness and/or syncope without any known cause were common complaints in patients in Group B but absent in patients in Group A. A prolonged DSACT could be of prognostic significance in sinus node dysfunction.