An electron microscopic observation has been made on the superior cervical ganglia of 2-day-old hamsters, in which most nerve cells were still immature and were named as primitive sympathetic nerve cells. They were small in size and 2 or 3 cells were grouped together to become many cell clusters. Only the periphery of each cell cluster was enclosed by the satellite cell cytoplasm. In these primitive nerve cells, only mitochondria, granular endoplasmic reticulum and Golgi apparatus were relatively prominent and the other cytoplasmic organelles were poorly developed. Axo-somatic synapses were found in the ganglia. Many unmyelinated nerve fibers were present in the ganglia. These nerve fibers were surrounded by the Schwann cell cytoplasm. Active mitotic divisions of the nerve cells were found in the superior cervical ganglia of 9-day-old hamsters and resulted in an increase in the number of nerve cells. The size of the nerve cells also increased and became young sympathetic nerve cells. In the 10-day-old hamsters, each young nerve cell was entirely enclosed by the satellite cell cytoplasm. The number of cytoplasmic organelles was increased considerably. Axo-somatic and axo-dendritic synapses were also observed in the ganglia of 10-day-old hamsters. The present study revealed that the developed cytoplasmic organelles in the young nerve cells of the 30-day-old young hamsters reached the level of the mature nerve cells in the ganglia of the adult hamsters. Multiple desmosomes were detected in the ganglia of both the young and adult hamsters.