In this study, the Purkinje neurons and granule neurons in the cerebellar cortex were studied in male Fischer 344 rats at 3, 9, 18, and 27 months of age. The numbers of Purkinje cells (PC) and granule cells (GC) in folia IV, VII, and X of the vermis were quantitated with the disector stereological technique. The numbers of PC and GC and the ratio of PC to GC were stable with advancing age. Measurements of the molecular layer, however, showed that this layer, the site of synaptic contact between GC and PC, decreased in volume with age.