Abstract. Objective: The primary aim of this study was to determine whether the incidence of bradyarrhythmias in preterm infants decreases with maturation. Study design: Cardiorespiratory waveforms in preterm infants with gestational age ≤32 weeks were acquired on a networked computer after they were weaned to room air or to supplemental oxygen through a nasal cannula. Waveforms for 24 hours were screened for bradycardia utilizing Neoscan® software. Bradyarrhythmias were classiﬁed as non-sinus atrial, junctional, ventricular, and other. Results: Sixteen preterm infants were evaluated longitudinally. Their mean birth weight and gestational age were 1030 ± 232 g and 28.2 ± 2.2 weeks, respectively. These infants were evaluated initially at a mean postmenstrual age (PMA) of 31 ± 1.2 weeks. Mean number of bradycardia episodes was 1.25 ± 0.95 per hour; of these 0.73 ± 0.78 had bradyarrhythmias. These infants were subsequently evaluated at a mean PMA of 34.4 ± 2.0 weeks. Mean number of bradycardia episodes had decreased to 0.85 ± 0.79 per hour (P < 0.05). Furthermore, the number of bradyarrhythmias decreased signiﬁcantly to 0.33 ± 0.37 per hour (P < 0.05). This decrease was also signiﬁcant when expressed as a ratio of arrhythmia episodes to total bradycardia episodes (P < 0.05). The vast majority of bradyarrhythmias were non-sinus atrial rhythm; junctional rhythm accounted for the remainder. Nine infants exhibited more than one type of arrhythmia. Conclusion: Non-sinus bradyarrhythmias occur frequently in preterm infants and their incidence decreases with postnatal maturation.