The expression of synaptophysin was immunohistocytochemically examined at the protein level in the cerebellar cortex of the neonatal rat with special reference to the development of granule cells. In the premigratory zone of the external granular layer, these cells expressed synaptophysin at an early stage of development, where it was localized in the trans-side cisterns of the Golgi apparatus and in nearby small vesicles. When the granule cells began to horizontally extend their axons in the upper molecular layer, the growth cones and expansions of parallel fibers already contained many small vesicles similar in shape and size to synaptic vesicles. These small vesicles were labeled with antibody against synaptophysin. Tubular structures were often found among the accumulations of synaptic vesicles, and their membranes were also labeled with synaptophysin. Both growth cones and expansions of parallel fibers were often observed to approach and to contact with the small process of Purkinje cell dendrites. The contact between the two components formed a junction with the characteristics of a synapse. The profiles of synaptogenesis began to be observed in the deeper molecular layer during a period from the 7th to the 9th days after birth.