In terms of physiological typing, three groups of synapses are recognized: excitatory, inhibitory and modulatory. Some neuroanatomical studies [14] have claimed that excitatory synapses possess spherical synaptic vesicles, whereas inhibitory synapses contain a predominance of flattened vesicles (Fig. 1-8). Other studies [15] have correlated this synaptic vesicular diversity with physiological data. In his study on the cerebellum, Gray [15] showed that neurons, with a known predominance of excitatory input on dendrites and an inhibitory input on the cell body, possessed two corresponding types of synapses; however, although this interpretation fits well in some loci of the CNS, it does not hold true for all regions. Furthermore, some workers consider that the differences between flat and spherical vesicles may reflect an artifact of aldehyde fixation or a difference in physiological state at the time of sampling. In light of these criticisms, it is clear that confirmation of the correlation between flattened vesicles and inhibitory synapses is required.