Electrophysiologic disorders in 17 patients with sick sinus syndrome (SSS) were assessed by recording of intracardiac electrograms, atrial overdrive pacing and extrastimulus technique. Significant suppression of the sinoatrial node (SAN) by overdrive pacing (maximum corrected SAN recovery time of longer than 560 msec) was noted in 14 or 16 patients studied. In nine patients, scanning with atrial extrastimuli, sinus rest was defined in all. In one patient there was a longer interpolation zone. Calculated sinoatrial conduction time (SACT) in individual patients varied considerably. The mean SACT was over 110 msec in 5 of 9 patients (56%). Sinus echo was demonstrated in 3; one manifested SAN- re-entrant tachycardia with rates of 72 to 77 beats/min. AV nodal echo was demonstrated in 3, two of them manifested AV nodal re-entrant tachycardia. Intracardiac electrograms revealed prolonged AV conduction time in 2 of 15 patients and prolonged His-Purkinje system conduction time in 2 of 17 patients studied. Two patients disclosed what we thought to be manifestation of intraatrial conduction disturbance. Both had considerable time interval between pacing impulse and atrial response. In one of them Mobitz type 1 and 2:1 intraatrial blocks were observed on atrial pacing and a possible internodal tract depolarization was also recorded. It is concluded that the electrophysiologic manifestations of patients with SSS cover a wide spectrum. The machanism of tachycardia can be due to either SAN or AV nodal re-entry.