It was investigated whether incidence of S-A block and A-V block (type I and II) observed in male Wistar rats shows a circadian periodicity. Under the 14/10 light-dark illumination schedule, circadian periodicity was demonstrated in the occurrence of bradyarrhythmias as well as slow wave sleep, paradoxical sleep locomotive activity and heart beats/hour. This circadian variation of bradyarrhythmia shows two major peaks. The highest peak coincides with the time zone immediately after the start of illumination, and the second peak coincides with the acrophase of the circadian variation in paradoxical sleep. The 25-h period of circadian rhythm of bradyarrhythmias was disclosed even under constant illumination. This circadian variation shows the disappearance of the early peak of two peaks observed in the light-dark schedule. These results show the existence of an endogenous circadian rhythm in bradyarrhythmias as well as in sleep-wakefulness and locomotive activity. This knowledge about the circadian rhythm of bradyarrhythmia must be incorporated into the effective treatment of arrhythmias.