The clinical trials unanimously ascribe the increased risk of cardiac events to apneic episodes. The effect of reoxygenation after apneic episode on the development of ventricular arrhythmias have not been studied in detail as a function of the light-dark cycle (LD cycle). We aimed to study the effect of reoxygenation on ventricular arrhythmia threshold (VAT) with respect to the LD cycle. Experiments were undertaken in female Wistar rats anaesthetised with ketamine/ xylazine (100mg/kg+15mg/kg, i.m.) after adaptation to a 12h:12h LD cycle for 4 weeks. The animals were ventilated artificially using a respiratory pump with the parameters of normal ventilation and reoxygenation as follows: V_T = 1 ml/100 g and respiratory rate, 50 breaths/min. The apneic episode was simulated by switching off the respirator for 2 minutes. The results clearly demonstrate the pro-arrhythmogenic effect of apneic episode regardless of the LD cycle. During reoxygenation, anti-arrhythmogenic effect (increase in VAT) was observed in the dark (active) part in contrast to pro-arrhythmogenic effect (decrease in VAT), which was demostrated during the light (nonactive) part of the day. These data suggest that heart rhythm disorders attributed to apneic episode can be attenuated or amplified by subsequent reoxygenation dependent on synchronization to the LD cycle.