We propose to study the completeness of functional development of sympathetic innervation of the canine heart at birth and relate any subsequent maturational changes to age. We also plan to examine if asymmetrical developmental cardiac sympathetic nerve (CSM) changes play a significant role in altering the cardiac electrophysiologic state and especially the arrhythmiogenesis in early life. By measuring refractory period (RP) changes during stimulation of the CSN in puppies 1 to 6 weeks of age we found that the functional sympathetic innervation of the heart is incomplete at birth. A peak in individual nerve activity at the second week, with asymmetrical decline at the third week, and then a gradual increase and leveling off in subsequent weeks was identified. This asymmetrical sympathetic development may form the basis for sympathetic imbalance in early life. Currently ventricular fibrillation thresholds are determined in the control state and during CSN stimulation while QT interval, T waveform, rate and rhythm changes are also observed. Findings in each age group will be compared and correlated with the already known RP changes in order to identify any relationships.