The rab3 protein is involved in the exocytosis of synaptic vesicles and secretory granules in the central nervous system and anterior pituitary cells.  We have elucidated the role of rab3B in growth hormone (GH) secretion and the mutual relationship between rab3B and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Confocal laser scanning microscopic observation of double immunohistochemical stainings for rab3B and GH revealed an increase of immunoreactivity for rab3B in growth hormone-releasing hormone (GHRH)-treated rats and a decrease of immunoreactivity for rab3B in and somatostatin (SRIF)-treated rats. Moreover, confocal laser scanning microscopic observation of double immunohistochemical stainings of synaptosomal-associated protein of 25 kDa (SNAP-25), syntaxin and rab3B revealed co-localization of rab3B and the SNARE proteins in GHRH-treated rats, and dissociation in SRIF-treated rats. These results suggest that rab3B plays an important role in GH secretion from the anterior pituitary cells, and that SNAP-25 and syntaxin act alongside rab3B in the functional regulation of GH secretion. Other reports have indicated that rho and rab proteins such as rab8B and rab27B are involved in the intracellular transport and secretion of pituitary hormones. Functional analyses of rab and rho protein families and the SNARE system are required to elucidate the precise mechanisms of the intracellular transport and secretion of pituitary hormones.