The response of six mRNAs (for prepro-corticotropin-releasing hormone, prepro-enkephalin, prepro-vasoactive intestinal polypeptide/peptide histidine isoleucine, prepro-neurotensin/neuromedin N, prepro-cholecystokinin, and prepro-tyrosine hydroxylase) was measured in the hypothalamic paraventricular and supraoptic nuclei after increasing periods of osmotic stimulation caused by the replacement of regular drinking water with hypertonic saline (up to five days) or by forced dehydration (up to three days). In addition, hematocrits and concentrations of corticosterone were determined after the different periods of osmotic stimulation and correlated with the effects on the content of the various mRNAs. The temporal response of the mRNAs within the paraventricular and supraoptic nuclei to osmotic stimulation was different within the three compartments of these nuclei. First, in response to overnight osmotic stimulation, magnocellular neurosecretory neurons increased their mRNA content for two molecules (prepro-corticotropin-releasing hormone and tyrosine hydroxylase). As the stimulus was maintained over the next two to four days, these cells accumulated the mRNAs for at least three other peptides (cholecystokinin, vasoactive intestinal polypeptide/peptide histidine isoleucine and enkephalin). Second, the response of peptide-coding mRNAs in parvicellular neurosecretory neurons of the paraventricular nucleus appeared to be slower; no changes could be measured after overnight stimulation. However, after a further two- to four-days of continued osmotic stimulation, the content of the mRNA coding for corticotropin-releasing hormone markedly decreased while that for cholecystokinin increased. No change in the content of the mRNAs coding for prepro-vasoactive intestinal polypeptide/peptide histidine isoleucine, enkephalin, and prepro-neurotensin/neuromedin N could be seen at any time after osmotic stimulation in parvicellular neurosecretory neurons. Third, increases in the content of mRNA coding for corticotropin-releasing hormone in the parvicellular neurons that provide descending projections from the paraventricular nucleus could only be detected after longer periods of osmotic stimulation. The effect of osmotic stimulation on plasma corticosterone concentrations was quickly apparent; plasma corticosterone concentrations were significantly elevated on the first morning after the beginning of salt-loading, and demonstrated the rapid effects of osmotic stimulation on the mechanisms controlling corticosterone release. These results show that the synthetic capability of cells in all three compartments of the paraventricular and supraoptic nuclei are modified by osmotic stimulation over different time scales, thereby allowing differential modulation of the neuroendocrine, autonomic, and behavioral components of the animal's response to disturbances in fluid homeostasis.(ABSTRACT TRUNCATED AT 400 WORDS)