In 61 patients sinus node function was tested by programmed (sinoatrial conduction time, SACT) and overdrive atrial pacing (sinus node recovery time, SRT). In the control group (N = 20), mean sinus cycle length was 773 +/- 140.2 msec, mean absolute SRT 1044 +/- 215.8 msec [corrected SRT (CSRT) 270 +/- 112.5 msec; mean +/- SD] and calculated SACT was 82 +/-19.2 msec. The upper limit of normal SACT was defined as 120 msec. In 41 patients with sinus node dysfunction, mean age (55 +/- 14.7 years), mean spontaneous cycle length during the study (1094 +/- 248.0 msec), the lowest heart rate observed (42 +/- 7.5 beats/min), maximal SRT (2110 +/- 1269.1 msec), maximal CSRT (1016 +/- 1182.8 msec), and calculated SACT (126 +/- 47.3 msec) were significantly longer than in the control group. Abnormalities of sinus node function, as evidenced by the degree of spontaneous bradycardia, SRT and calculated SACT were more frequent in patients with bradycardia-tachycardia syndrome or spontaneous sinoatrial block than in those exhibiting isolated sinus bradycardia. We conclude 1) that in patients with sinus node dysfunction both sinus node automaticity and sinoatrial conduction may be abnormal, and 2) that overdrive and programmed premature atrial stimulation can separate patients with sinus node dysfunction according to their clinical presentation.