The superior cervical sympathetic ganglion of the rat receives its preganglionic afferent innervation through the cervical sympathetic trunk, and sends most of its postganglionic axons through two major nerves, the internal (ICN) and external carotid nerves (ECN). In the present study, the ICN alone or both the ICN and the ECN were cut in neonatal and adult rats. Two months after these lesions, ganglionic neurons, synapses and preganglionic axons were counted and compared with unoperated control values. After cutting the ICN alone in neonatal rats, ganglionic neurons were reduced in number by 70% and synapses were reduced by 50%, but there was no change in the number of preganglionic axons. Cutting both the ICN and ECN in neonates resulted in an 88% reduction of ganglionic neurons and an 83% reduction of synapses. In this case there was a 63% reduction in the number of preganglionic axons. After cutting either the ICN alone or both the ICN and the ECN in neonates, there was a hyperinnervation (increased number of synapses) of the remaining ganglionic neurons. In the adult rat, cutting either the ICN alone or both the ICN and ECN resulted in a smaller loss of ganglionic neurons, and there was no loss of preganglionic axons. There was no hyperinnervation of surviving neurons in adult rats. Thus, the response by preganglionic axons to a reduced number of ganglionic neurons differs in the neonate and adult rat. In the developing animal, the degenerative response to injury is much more severe than in the adult, but the reorganizational response is also greater.