The effects of maternal malnutrition on the formation of cerebellar fissures was studied in foetal rats. The rats were given a diet containing either 14 (normal) or 7 (protein-deprived) per cent protein by weight from two weeks before conception and throughout gestation. The first day with a sperm-positive vaginal smear was designated as Embryonic day 1 (E 1). Foetuses were examined on days E 19 to 22. In seventy-five foetuses, a sagittal slice of the cerebellum, 3-4 mm thick was embedded in Sorvall embedding medium. Sagittal sections, 2 micron thick were cut at levels 100 micron apart through the specimen. Sixty-three foetuses were examined for presence of cerebellar fissures only after filling their vascular system with ink-gelatin and cutting serial sagittal frozen sections, 100 micron thick. A morphological comparison of the foliation in normal and malnourished foetuses suggested a slight delay in the malnourished ones, albeit the earliest fissures were visible on the same day as in normal foetuses. The existence of a developmental alteration due to malnutrition was demonstrated by a decrease in the length of the pial surface and a surface folding index day on E 22. The alteration is discussed in relation to previously reported changes in cerebellar development during malnutrition, such as delayed Purkinje cell formation and alterations in proliferation/differentiation in the EGL.