Ras-like small GTP-binding proteins of the Rab family regulate trafficking of the secretory or endocytic pathways. Rab3 proteins within the Rab family are expressed at high levels in neurons and endocrine cells, where they regulate release of dense-core granules and synaptic vesicles (SVs). Rab3A is present as either the soluble or the SV membrane-bound form in neurons that are dependent on the GDP- or GTP-bound states respectively. GDP dissociation inhibitor (GDI) is known to induce the dissociation of Rab3A from synaptic membranes when GTP is depleted. In an earlier study, Ca(2+)/calmodulin (CaM) was also shown to dissociate Rab3A from synaptic membranes by forming an equimolar complex with Rab3A in vitro. We have examined a possible role for Ca(2+)/CaM in modulating both the binding of guanine nucleotides to Rab3A and the GTPase activity of Rab3A. The basal level of Rab3A GTPase activity was not affected by an association with Ca(2+)/CaM. Ca(2+)/CaM-Rab3A complex that was formed in synaptic membranes was able to bind guanine nucleotides, whereas the Rab3A-GDI complex could not. In addition, Ca(2+)/CaM led to the replacement of the GDP molecule in the Rab3A-GDI complex with GTP in Rab3A. Taken together, these results suggest that CaM may have a role in stimulating GTP binding to Rab3A that is complexed with GDI, which leads to the formation of an active GTP-bound form of the Rab3A-Ca(2+)/CaM complex.