It is well established that heart rate (HR) changes during the course of postnatal development in mammalian species. In general, the HR of large animals, including humans, rabbits, and dogs, decreases with development. The HR of small animals such as mice and rats has been reported to increase after birth. These contrasting HR changes may be related to the size of the body and heart, and may be purposely designed to most effectively propel the blood around the body in various mammalian species. Nevertheless, the physiological significance and mechanisms underlying these postnatal changes in HR remain poorly understood. In the present study, we examined the mechanisms of postnatal change in murine HR using in vivo non-invasive measurement of HR and electrophysiological analysis of ion channels in isolated sinoatrial node (SAN) cells. Non-invasive measurement of murine HR was accomplished using a piezoelectric transducer (PZT) sensor. Mice were simply placed on the PZT sensor, and the heart sound signal was extracted and used for calculating HR. In this method, HR was 323 ± 17 bpm at P0, and increased daily during 2 weeks after birth. The HR at P14 was 685 ± 18 bpm. Under pharmacological blockade of the autonomic nervous system, the HR of approximately 300 bpm was largely constant at P0-5, and then increased daily to 500 bpm at P14. In intracellular potential recordings, the spontaneous beating rate of the SAN tissue preparation was traced to similar curve of in vivo HR under pharmacological blockade. These findings indicate that the postnatal increase in HR is derived from increased sympathetic influence that becomes apparent immediately after birth, and increased intrinsic activity of SAN cells that emerges 5-6 days after birth. In spontaneous action potential recordings in isolated SAN cells by the whole cell patch clamp method, the beating rate and diastolic depolarization rate of SAN cells was significantly less in neonatal mice than in adult mice. Isolated pacemaker cells possessed similar morphological and electrophysiological characteristics for sinoatrial pacemaker cells. The ICaL was activated at more positive potentials and the current amplitude was smaller in newborn SAN cells than in adult cells, however no significant difference was detected in the If and ICaT density and kinetics. Quantitative PCR using the SAN tissue preparations revealed no statistical differences in the expression levels of CaV1.2, CaV1.3, CaV3.1, CaV3.2, and HCN4 between neonate and adult SAN tissues. We conclude that the postnatal increase in HR is caused by increased sympathetic influence and the intrinsic activity of SAN cells, and that the different conductance and kinetics of ICaL may be involved in the postnatal increase in pacemaker activity.