In situ hybridization was used to study the mRNA levels for vasopressin, galanin, secretogranin II and carboxypeptidase H in salt-loaded and Brattleboro rats. These animals represent an in vivo model for the chronic stimulation of the hypothalamo-neurohypophyseal neurons. As shown by immunelectron microscopy secretogranin II is co-stored with vasopressin in these neurons. In salt-loaded rats the levels of mRNA for vasopressin, galanin and secretogranin II are increased in the paraventricular and supraoptic nuclei. Analogous changes were observed for Brattleboro rats with the exception of the vasopressin message which was decreased in these animals. The secretogranin II message was also increased in neurons which do not contain the vasopressin mRNA, i.e. in magnocellular neurons of the lateral hypothalamus and in the subfornical organ. Carboxypeptidase H message was also found in the paraventricular and supraoptic nuclei and in the subfornical organ; however, in both models the changes in mRNA in these nuclei were much lower than those observed for the secretory peptides or non-existent. We conclude that chronic stimulation of vasopressin neurons leads to a concomitant up-regulation of the biosynthesis of neuropeptides and secretogranin II. We suggest that the secretogranin II message might be a useful general marker for identifying chronically stimulated neurons.