It is obvious that the development of the nervous system is marked by the progression from immaturity of function to the acquisition of neurotransmission and integrated control of synaptic activity. However, there is an increasing realization that neural function in the fetus and neonate also serves specialized needs which are particular to development, and that the onset of maturity equally represents the loss of these unique patterns of neural activity. Nowhere is this more evident than in the sympathetic nervous system and its endocrine counterpart, the adrenal medulla. This review will detail recent work which demonstrates how catecholamines released first by the adrenal, and later by neurons, mediate the transition from fetal to neonatal physiological function as well as the subsequent programming of postsynaptic reactivity and cellular differentiation in target tissues. The rat has been chosen for study as this species is altricial, and therefore develops neural function relatively late. Thus, in the rat many fetal neural characteristics persist into the postnatal period.