Twenty patients of mean age 66.2 years, with suspected sinus node dysfunction, underwent extensive electrophysiologic study. Sinus bradycardia (18), sinus pauses (3), and sinoatrial block (1) were identified in their ECGs prior to study. Also 11 patients had some abnormality of atrioventricular nodal and/or intraventricular conduction prior to study. At the time of electrophysiological study, 10/20 patients (50%) had a mean cycle length exceeding 1000 msec, and mean P-V interval exceeded 210 msec in 7/20 (35%). The estimated "sinoatrial conduction time" exceeded 215 msec in 6/16 (38%) patients. The maximum first escape cycle following pacing at six different rates exceeded a value equal 1.3 X the mean value of the control cycle length + 101 msec (slope of regression line + Y intercept + 1 SD) in 13/9 (68%) patients. Nineteen patients received 1 mg atropine intravenously and mean cycle length decreased by 19%, from 891 +/- 175.8 msec to 718 +/- 182.9 msec. Graded infusion of isoproterenol was employed in 19 patients; four patients required an infusion rate greater than 28.3 ng/kg/min to produce a 20% decrease in spontaneous sinus cycle length. These data would indicate that a variety of interventions are required to characterize the disturbance of sinus node automaticiy and sinoatrial conduction in patients with sinus node dysfunction.