The differentiation of granule cells in organotypic cultures of rat hippocampus was studied by means of the Golgi/electron microscopic (EM) technique. Like in vivo, the granule cells have a small round or avoid cell body which gives rise to apical dendrites densely covered with spines. However, the apical dendrites of the cultured granule cells are more horizontally oriented than in the normal fascia dentata where they form a cone-shaped dendritic arbor. Granule cells in slice cultures occasionally have basal dendrites invading the hilar region. Electron microscopic examination revealed many synaptic contacts on identified apical and basal dendrites of the gold-toned granule cells in culture. This suggests that a considerable synaptic reorganization takes place since all extrinsic afferents normally innervating the granule cells are lost. Several granule cells displayed deep infoldings of their nuclei which are known from in vivo studies to be a characteristic feature of non-granule cells in this region. i.e. basket cells. The presence of basal dendrites and nuclear infolding indicates an increased variability of this cell type which is situ displays a rather stereotyped morphology.