The unipolar brush cells are excitatory, cerebellar granular layer interneurons that receive mossy fiber input on their dendritic brushes in the form of a giant glutamatergic synapse. We investigated the postnatal development of the brush of the unipolar brush cell in lobules IX and X by light microscopy and defined the maturation of mossy fiber-unipolar brush cell synapses and mossy fiber-granule cell synapses by electron microscopy using calretinin immunocytochemistry to identify unipolar brush cells. During the first postnatal week, unipolar brush cells possessed one or two short, branched dendrites. The brush differentiated primarily during the successive 21 postnatal (P) days, during which it underwent progressive maturation. This developmental process was subdivided into stages 1-4, which were descriptively termed protodendritic unipolar brush cell (P2-12), filopodial brush (P12-16), intermediate brush (P16-21), and dendriolar brush (P21-28) stages. Electron microscopic measurements of individual mossy fiber-unipolar brush cell and mossy fiber-granule cell synaptic junctions were made at P12, 16, 21, and 28. While the average length of mossy fiber-unipolar brush cell synapses increased during development, that of mossy fiber-granule cell synapses decreased. Comparisons of the lengths of mossy fiber-unipolar brush cell and mossy fiber-granule cell synapses demonstrated that mossy fiber-unipolar brush cell synapses were longer on average than mossy fiber-granule cell synapses for all ages. Frequency distribution histograms also showed that the percentage of mossy fiber-unipolar brush cell synapses longer than 0.5 microm was lower in the pooled P12-P16 groups than in the pooled P21-P28 groups (8 versus 20%). In contrast, mossy fiber-granule cell synapses longer than 0.5 microm were a small minority at P12, 16, and 21, and occurred rarely at P28. The present study indicates that mossy fiber-unipolar brush cell synapses increase in length with the differentiation of the brush dendrioles, while that of mossy fiber-granule cell synapses decrease with differentiation of the granule cell dendritic claws. The finding that mossy fiber-unipolar brush cell synapses were generally longer than mossy fiber-granule cell synapses may indicate that the properties of the postsynaptic targets play a major role in shaping synaptic appositions within cerebellar glomeruli.