The cerebellar cortices of foetal rats were examined for structural changes occurring prior to the formation of fissures. Female rats were mated overnight and the first day on which a sperm-positive vaginal smear was found was designated as embryonic day 1 (E1). Birth of rats generally occurs on day E23. Foetuses from days E19 to 22 were fixed by perfusion followed by immersion, using a mixture of glutaraldehyde and paraformaldehyde, and their cerebella embedded in plastic. The cerebella of thirty-five foetuses were sectioned sagittally at intervals of 100 micron, and those of five were sectioned coronally. A continuous Purkinje cell layer was formed during the examination period by fusion of four transverse bands. The study revealed formation of pre-migratory granule cells in the EGL of foetal rats. The granule cells were orientated in the coronal plane and were seen only where Purkinje cells were present beneath the EGL. A local increase in pre-migratory granule cells and indentation of the Purkinje cell layer were early signs of foliation and occurred prior to the formation of a fissure. The subsequent deepening of the fissures was due to the outward expansion of lobules, and the bottom of the different fissures retained their spatial relationship. The findings show that the expansion of the cerebellar surface is tangential before and after the formation of a fisure. In order to indent the surface, the direction of the expansion must change temporarily. This reorientation of the expansion appears to be related to the cortical changes which have been described and marks the onset of foliation.