Synapse formation during postnatal development of the superior cervical sympathetic ganglion was studied in rats after neonatal ganglionic denervation, when reinnervation was either permitted or prevented. In both groups of operated animals, 90% of the synapses were lost by the fourth postoperative day. In the rats in which reinnervation was permitted, restoration of synapse numbers began by one month after surgery and reached 50% of control by two months. There was no further synapse restoration after this time. In the animals in which reinnervation was prevented, synapse numbers increased, but at all times were approximately 10% of controls. Thus the intrinsic ganglionic synapses underwent their normal postnatal developmental increase in number, but did not sprout to any significant degree in response to the massive deafferentation caused by removal of the preganglionic input. Vacant postsynaptic membrane thickenings (those not apposed by presynaptic terminals) appeared in both experimental groups. There was no significant loss of these vacant thickenings in either group over the course of 3 months. Therefore, the vacated postsynaptic sites do not appear to be recontacted during reinnervation, while the reinnervating axons appear to cause the formation of new postsynaptic sites on the ganglion cells.