The GTPase Rab3A has been postulated to cycle on and off synaptic membranes during the course of neurotransmission. Moreover, a Rab guanine nucleotide dissociation inhibitor has been shown to cause Rab3A to dissociate from synaptic membranes in vitro. We demonstrate here that Ca2+/calmodulin also can cause Rab3A to dissociate from synaptic membranes in vitro. Like Rab guanine nucleotide dissociation inhibitor, it forms a 1:1 complex with Rab3A that requires both the lipidated C terminus of Rab3A and the presence of bound guanine nucleotide. In addition, a synthetic peptide corresponding to the Lys62-Arg85 sequence of Rab3A can prevent the dissociating effect of each protein and disrupt complexes between each protein and Rab3A. However, Ca2+/calmodulin's effect differs from that of Rab guanine nucleotide dissociation inhibitor not only in being Ca2+-dependent but also in having a less stringent requirement for GDP as opposed to GTP and in involving a less complete dissociation of Rab3A. The functional significance in vivo of Ca2+/calmodulin's effect remains to be determined; it may depend in part on the relative amounts of Ca2+/calmodulin and Rab guanine nucleotide dissociation inhibitor that are available for binding to Rab3A in individual, activated nerve termini.