The differentiation of cerebellar glomeruli was investigated by quantitative electron microscopy, starting with the period at which mossy fibers made their first appearance. Two developmental stages could be delineated in the maturation of the mossy fiber-granule cell synapse. 1. A primary growth stage (postnatal days 6--15) characterized by the rapid enlargement of mossy rosettes and the intense proliferation of post-synaptic dendrites. The synaptic perimeter of mossy terminals, i.e. percentage of membrane surface occupied by synaptic junctions, exhibited a simultaneous, rapid increase in that stage, reaching a peak at postnatal day 15. 2. Establishment and stabilization of differentiated glomeruli (15th--45th day). Because the size of mossy rosettes did not change in this period, the increase of glomerular size was due exclusively to the continuing multiplication of postsynaptic dendrites. The characteristic feature of this stage was the massive elimination o synaptic junctions. The synaptic perimeter of 14.4% at day 15 decreased to 5.7% by day 30. Since the size of individual synaptic junctions and the size of mossy terminals did not decrease while the number of postsynaptic dendrites even increased during the same period, the elimination of synaptic junctions represents a net loss of the synaptic perimeter of mossy terminals. The quantitative analysis suggests that the stabilization of the synaptic perimeter of mossy rosettes at about 6% is due to the elimination during the second developmental stage of immature synaptic junctions, produced in excess during the first growth phase. Also, the observation that synapse elimination and the subsequent stabilization of synaptic perimeter occurs in spite of a steady increase of available postsynaptic dendrites is indicative that the standard 6% value of synaptic perimeter is defined by the presynaptic mossy terminal itself. On the basis of these observations, it is also proposed that elimination of synaptic junctions may well occur without the concomitant disappearance of presynaptic and/or postsynaptic neuronal processes.