Longitudinal Cohort Study: Electrocardiogram and blood pressure were taken biannually along with history taking, physical examination and other laboratory studies. 6,690 subjects were included in this study in whom at least 7 out of 9 biannual check up examinations were performed. At the end of the study period, their ages ranged from about 40 to 90 years. The incidence of atrial fibrillation was about 0.2% in the forties and early fifties and increased to 0.6% by the late fifties. The incidence of atrial fibrillation was then increased almost linearly up to 2.5% at the end of the eighties. Likewise, incidence of CRBBB and LBBB was also increased with age even after the age of sixty; 1.0% at the fifties to 7.5% at the eighties in CRBBB and 0.05% at the fifties to 1.4% at the eighties in LBBB, respectively. Holter ECG Study: Holter ECG was recorded in 164 healthy subjects aged 14 to 87 years in whom no arrhythmias were found in the routine 12 leads ECG at entry to the study. In 96.9% of the subjects APC was recorded in the 24-hour Holter ECG irrespective of their age. The total number of APCs in 24 hours significantly increased with age, especially after age sixty. The incidence of couplet or short run APCs was 21.4% under age sixty and 74.2% above age sixty. Electrophysiologic Studies in Patients with Paroxysmal Atrial Fibrillation (Paf): Repetitive atrial firing (RAF) elicited by premature atrial stimulation, and prolonged intra-atrial electrogram (PAE) with multiple (more than 7) spikes recorded during sinus rhythm were taken as indicators of atrial vulnerability. RAF and PAE was observed in more than 60% of Paf patients with or without sick sinus syndrome (SSS), but only approximately 25% in the control group (without SSS). It was also noted that in patients with SSS, who were generally of old age, RAF was observed in about 63% even without Paf. These results suggest that the atrial vulnerability might be an expression of the common electrophysiologic properties of the atrial muscle in the elderly with atrial arrhythmias and/or SSS. Syncopal Episodes due to Transient Severe Hyperkalemia in the Elderly: Two patients with mild to moderate chronic renal failure developed transient severe hyperkalemia (9.9, 7.3 mEq/L, respectively), severe sinus bradycardia with sinus arrest and syncopal episodes. These transient findings almost completely improved in a few days by kayexatete and bicarbonate, and temporary back-up atrial pacing. Hyperkalemia was disproportionately severe compared to BUN and serum creatinine.(ABSTRACT TRUNCATED AT 400 WORDS)