There is accumulating evidence that small GTPases of the rab family regulate intracellular vesicle traffic along biosynthetic and endocytotic pathways in eukaryotic cells. It has been suggested that Rab3a, which is associated with synaptic vesicles in neurons and with secretory granules in adrenal chromaffin cells, might regulate exocytosis. We report here that overexpression in PC12 cells of Rab3a mutant proteins defective in either GTP hydrolysis or in guanine nucleotide binding inhibited exocytosis, as measured by a double indirect immunofluorescence assay. Moreover, injection of the purified mutant proteins into bovine adrenal chromaffin cells also inhibited exocytosis, as monitored by membrane capacitance measurements. Finally, the electrophysiological approach showed that bovine chromaffin cells which were intracellularly injected with antisense oligonucleotides targeted to the rab3a messenger exhibited an increasing potential to respond to repetitive stimulations. In contrast, control cells showed a phenomenon of desensitization. These results provide clear evidence that Rab3a is involved in regulated exocytosis and suggest that Rab3a is a regulatory factor that prevents exocytosis from occurring unless secretion is triggered. Furthermore, it is proposed that Rab3a is involved in adaptive processes such as response habituation.