Double-peaking circadian variation in the occurrence of sustained supraventricular tachyarrhythmias. We studied 251 patients less than or equal to 65 years of age admitted for treatment of symptomatic supraventricular tachyarrhythmia to assess whether these arrhythmias begin evenly throughout the day or manifest circadian variation in occurrence. The arrhythmias included 152 episodes of atrial fibrillation, 50 episodes of supraventricular reentry tachycardia, 30 episodes of atrial flutter, and 19 cases of ectopic atrial tachycardia. A total of 209 patients could tell the exact time their symptoms had started. In 38 of them (18%), the arrhythmia had begun between midnight and 6:00 AM, in 63 (30%) between 6:01 AM and noon, in 46 (22%) between noon and 6:00 PM, and in 62 (30%) between 6:01 PM and midnight. This distribution differed significantly from uniform occurrence (chi square 8.7, p less than 0.05). Fifty patients were using beta-adrenoceptor blocking agents when the arrhythmia occurred. Compared with the other 159 patients, they had no morning surge of arrhythmias (20% versus 33.3% of episodes between 6:01 AM and noon), but instead a much higher incidence at night (34% versus 13.2% of episodes between midnight and 6:00 AM) (chi square 14.4, p less than 0.005). We conclude that the frequency of onset of sustained supraventricular tachyarrhythmias varies with the time of day, showing nearly equal peaks in the morning and in the evening and a trough at night. The modifying effect of beta-adrenoceptor blockage suggests that many morning arrhythmias are of adrenergic origin while other, probably vagal arrhythmogenic mechanisms, prevail at night. 