Exocytosis is regulated by proteins which interact to promote docking and fusion of secretory granules with the plasma membrane. We have used in situ hybridization to study the mRNA expression for vesicle- associated membrane protein (VAMP) isoforms VAMP-1 and VAMP-2, synaptosomal- associated protein of 25-kDa (SNAP-25) isoforms SNAP-25a and SNAP-25b, mammalian homologue of unc-18 (munc-18) and Hrs-2 in neurosecretory neurons of the magnocellular paraventricular (PVN) and supraoptic (SON) nuclei of normal and osmotically challenged animals. In PVN and SON neurons of normal animals, strong labeling was demonstrated for VAMP-2 and SNAP-25a mRNA, whereas VAMP-1 or SNAP-25b mRNA could not be detected. Salt-loading (2% NaCl as drinking water), an animal model which increases the expression and secretion of hormones from hypothalamic magnocellular neurons, resulted in significantly increased mRNA levels for VAMP-2 (36%, 28%), munc-18 (74%, 68%) and SNAP-25a (59%, 77%) in the PVN and SON, respectively. There was no significant increase in Hrs-2 mRNA levels in the PVN, whereas a significant increase (22%) was observed in the SON. In the posterior pituitary, immunohistochemistry showed a marked decrease in numbers and intensity of vasopressin-immunoreactive (-IR) nerve endings after salt-loading. There were no obvious changes in numbers or intensity of VAMP-2-, munc-18-, Hrs-2- or SNAP-25-IR fibers. Large varicosities containing VAMP-2- and Hrs-2 immunocreactivity were seen in salt-loaded animals. The results show isoform-specific mRNA expression in neurosecretory neurons and an increased mRNA expression of proteins participating in the molecular regulation of exocytosis during an experimental situation characterized by increased secretion.