Beginning prenatally and during the first week after birth, there is normally a loss of axons in rat cervical sympathetic trunk. To test the hypothesis that this spontaneous axonal loss represents a natural process whereby an excessive number of immature preganglionic axons in the cervical sympathetic trunk adapts to the neuronal population in the superior cervical ganglion, the number of nerve cells in the superior cervical ganglion was reduced in newborn rats by administration of nerve growth factor antiserum, 6-hydroxy-dopamine or postganglionic anxotomy. Quantitative ultrastructural studies of these animals at later stages of development revealed that, with each method, the number of preganglionic axons and Schwann cells was reduced to nearly one-third of normal. These findings indicate that the superior cervical ganglion plays an important role in the development of the cervical sympathetic trunk. Removal of ganglionic cells causes a retrograde loss of preganglionic fibres. This process probably represents an exaggeration of the normal mechanism for elimination of redundant axons. Because the changes in axonal numbers are associated with similar reductions in the number of Schwann cells, it can also be concluded that postnatal Schwann cell proliferation is influenced by axonal populations.