Twenty-four-hour recordings of electrocardiogram (ECG) were obtained from 1528 subjects. Sick sinus syndrome (SSS) was observed in 34 and advanced or complete AV block (CAVB) in 13 subjects. During the period studied 21 patients had died, who showed complicated ventricular premature contractions in 9 (multiform 6, short run 2, repetitive 2). Seventy-three patients with artificial cardiac pacing who were admitted at three different institutions were analyzed. The discrepancy of degree of block was observed between His bundle electrocardiographic findings and 24-hour ECG recordings. In 15 patients there was no correlation between sinus node recovery time following overdrive suppression and pauses following the termination of tachycardia (r = 0.0896, N. S.). In 49 normal subjects 24-hour heart rate ranged from a high of 170 to a low of 36 beats/min. Longest R-R interval ranged from 2.08 to 0.86 sec (mean 1.25 +/- 0.22 sec). The critical level of longest R-R interval to differentiate between normal group and groups with SSS or CAVB was about 1.6 sec. Above results suggest that Holter monitoring permits a precise diagnosis of arrhythmias and that both HBE and Holter monitoring should be used for the determination of pacemaker implantation especially in those with syncope or dizziness but without evidence of marked bradycardia.