The purpose of this study was to assess prospectively the circadian distribution of spontaneous paroxysmal supraventricular tachycardia (PSVT) in drug-free subjects with no previous history or symptoms and signs of concomitant heart or lung disease. Of 112,424 presumably diurnally active patients admitted to the Emergency Department of a city hospital during a 2-year period (1990-1991), a total of 185 patients were screened with these characteristics. Time of symptom onset was exactly recordable in 177 (75 men and 102 women). Analysis of variance documented a higher incidence in the morning-afternoon hours. Cosinor analysis, although not a perfect method for the time series analysis, verified circadian rhythmicity with afternoon peak times. Our findings suggest that a circadian pattern in intrinsic electrical instability of the heart conduction system exists irrespective of the circadian fluctuations in the pathophysiologic mechanisms of the cardiovascular or lung diseases most frequently associated with PSVT itself.