The morphology and degree of maturation of intrafissural colony-forming ectopic granule cells (EGCs) were investigated in the cerebella of normal adult rats. These EGCs exhibit a notable degree of morphological maturity, with 2-5 short dendrites radiating from a globular soma, typical dendritic claw-like terminals and descending axons. The EGCs receive the afferent input of mossy fiber terminals and their axons also establish homologous synapses with the postsynaptic dendritic spines of the Purkinje cells. Although these EGCs exhibit the typical morphology of mature granule cells, the morphometric analysis shows, however, a significant reduction in the number of dendrites (mean 3.30 +/- 0.60) and 'claws' (mean 3.66 +/- 0.12) in the EGCs with respect to the controls (mean 3.85 +/- 0.73 and 4.53 +/- 0.13, respectively). These differences may be indicative of a slight numerical mismatching between EGCs and afferent mossy fibers, which limit the input that the ectopic neurons receive. The karyometric study also showed that the average nuclear area in EGCs was significantly smaller (mean 19.38 +/- 0.24 micron2) than in controls (mean 21.88 +/- 0.24 micron2), suggesting a positive correlation between nuclear size and the number of postsynaptic dendritic terminals available in the granule cells. The influence of some morphogenetic factors on the degree of maturity attained by these ectopic neurons is discussed, particularly the interactions of mossy fibers and astrocytes with maturing EGCs.