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The size distribution of synapses in the cerebellar cortex of the cat was defined on 0.5-micron semithin sections stained with ethanolic phosphotungstic acid (E-PTA). The surface area (SA) of synaptic grids was measured and the number of dense projections per grid (NDP) was counted. The results show large differences in mean values between molecular and granular layer. Within the molecular layer the differences in mean values at different levels below the pial surface were small; however, the frequency distributions differed significantly. In the granular layer a confined unimodal frequency distribution of SA and NDP was observed (mean NDP, 7.02 +/- 0.10), in the molecular layer a considerable variation in the size of the synaptic discs was observed (mean NDP, 20.40 +/- 0.43). Only a small percentage of the synaptic discs have less than 5 or more than 47 DPs. The sharply defined differences in synaptic size between the granular and molecular layer and the smaller differences within the granular and molecular layer are discussed in the context of the congruity hypothesis of Chan-Palay.