Age-related transformations and nature of short-term positive and negative oscillations of cardiac contraction rhythm were studied in comparison with periodical spontaneous somatomotor activity bursts in rats for the first 3 weeks after birth. It has been established that both accelerations and decelerations can be independent, precede motor excitation or appear as a consequence of muscle contraction. The main role in genesis of decelerations is played by parasympathetic mechanisms whose action is blocked by atropine. Activation of vagus cardiomotor centers can ether occur spontaneously by producing independent declerations or appear as reaction to motor excitation. Genesis of accelerations is connected with activity of sympathetic nervous system, as they are inhibited under conditions of desympathization or block of adrenoreceptors. The main role in realization of accelerations is played by somatosympathetic reflex, but connection with motor activity can also be effected by the principle of pre-tuning to expected movement.