Synapsins regulate synaptic transmission by controlling the reserve pool of synaptic vesicles. Each of the three mammalian synapsin genes is subject to alternative splicing, yielding several isoforms whose roles are unknown. To investigate the function of these isoforms, we examined the synaptic effects of introducing each isoform into glutamatergic cultured hippocampal neurons from synapsin triple knock-out mice. Remarkably, we found that synapsin IIa was the only isoform that could rescue the synaptic depression phenotype of the triple knock-out mice; other isoforms examined, including the well-studied synapsin Ia isoform, had no significant effect on the kinetics of synaptic depression. The slowing of synaptic depression by synapsin IIa was quantitatively paralleled by an increase in the density of reserve pool synaptic vesicles, as measured either by fluorescent tagging of the vesicle protein synaptobrevin-2 or by staining with the styryl dye FM4-64 [N-(3-triethylammoniumpropyl)-4-(6-(4-diethylamino)phenyl)-hexatrienyl)pyridinium dibromide]. Our results provide further support for the hypothesis that synapsins define the kinetics of synaptic depression at glutamatergic synapses by controlling the size of the vesicular reserve pool and identify synapsin IIa as the isoform primarily responsible for this task.