A quantitative approach to the nuclear ultrastructure of cerebellar granule cells is described here. The study was made using conventional electron microscopy from cerebellar cortices of adult rats by means of a semiautomatic image analyzer. The basic observation is that the nuclei of mature granule cells constitute a homogeneous population in terms of morphometric and stereologic data; in fact, the volume density of condensed chromatin within the nuclei remains practically constant in all nuclear sections. These results seem to indicate the existence of a cell-specific nuclear morphometric phenotype which might be considered as an effective criterion for the typification of this cellular lineage.