A reliable basal heart rate (HR) measurement in freely moving newborn mice was accomplished for the first time by using a novel noninvasive piezoelectric-transducer (PZT) sensor. The basal HR was ~320 beats/min at postnatal day 0 (P0) and increased with age to ~690 beats/min at P14. Contribution of autonomic control to HR was then assessed. Sympathetic blockade with metoprolol significantly reduced the basal HR at both P6 (mean ± SE; -236 ± 23 beats/min) and P12 (-105 ± 8 beats/min) but atropine was without effect, indicating the predominant tonic adrenergic stimulation and absence of vagal control for the basal HR in newborn mice. In contrast to stable basal HR during 5-min recording, HR measured by ECG (ECG-HR) was markedly decreased due to the restraint stress by attaching ECG electrodes, with accompanying freezing behavior. The ECG-HR lowered and further decreased gradually during 5 min (slow cardiodeceleration) at P0-P3 and it rapidly decreased and gradually recovered within 5 min (transient bradycardia) at P9-P14. The response was not uniform in P4-P8 mice: they showed either of these two patterns or sustained bradycardia (9-29%) and the number of mice that showed the transient bradycardia increased with age (30 to 100%) during the period. Studies with autonomic blockade suggest that the slow cardiodeceleration and the transient bradycardia is mediated mainly by withdrawal of adrenergic stimulation and phasic vagal activation, respectively, and the autonomic control of HR response to restraint stress is likely to change from the withdrawal of adrenergic stimulation to the phasic vagal activation at different stages during P4-P8 in individual mice. The PZT sensor may offer excellent opportunities to monitor basal HR of small animals noninvasively.