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We investigated the roles of two Rab-family proteins, Rab3a and Rab5a, in hippocampal synaptic transmission using real-time fluorescence imaging. During synaptic activity, Rab3a dissociated from synaptic vesicles and dispersed into neighbouring axonal regions. Dispersion required calcium-dependent exocytosis and was complete before the entire vesicle pool turned over. In contrast, even prolonged synaptic activity produced limited dispersion of Rab5a. A GTPase-deficient mutant, Rab3a (Q81L), dispersed more slowly than wild-type Rab3a, and decreased the rate of exocytosis and the size of the recycling pool of vesicles. While overexpression of Rab3a did not affect vesicle recycling, overexpression of Rab5a reduced the recycling pool size by 50%. We propose that while Rab3a preferentially associates with recycling synaptic vesicles and modulates their trafficking, Rab5a is largely excluded from recycling vesicles. [ABSTRACT FROM AUTHOR]