Rab3a is a small GTPase that binds selectively to secretory vesicles and switches between active, GTP-bound and inactive. GDP-bound conformations. In yeast, Rab and SM-genes interact genetically to promote vesicle targeting/fusion. We tested different Rab3a conformations and genetic interactions with the SM-gene munc18-1 on the docking function of Rab3 in mammalian chromaffin cells. We expressed Rab3a mutants locked in the GTP- or GDP-bound form in wild-type and much 18-1 null mutant cells and analyzed secretory vesicle distribution. We confirmed that wild-type Rab3a promotes vesicle docking in wild-type cells. Unexpectedly, both GTP-and GDP-locked Rab3a mutants did not promote dockling. Furthermore, wild-type. Rab3a did not promote docking in munc18-1 null cells and GTP and GDP-Rab3a both decreased the amount of docked vesicles. The results show that GTP-and GDP-locked conformations do not support a Munc 18-1 dependent role of Rabta in docking. This suggests that nucleotide cycling is required to support docking and that this action of Rab3a is upstream of Munc 18-1.