In　this　study　we　try　to　simultaneously　investigate　the　response　of　neurons　and　astrocytes　of　rats　following　hyperosmotic　stimulation　and　test　the　possibility　that　the　reciprocal　pathways　between　medullary　visceral　zone　(MVZ)　and　hypothalamic　paraventricular　nucleus　(PVN)　or　supraoptic　nucleus　(SON).　Hyperosmotic　pressure　animal　model　was　established　by　administering　3%　sodium　chloride　as　drinking　water　to　rats.　The　distribution　and　expression　of　the　HRP　retrogradely　labeled　neurons,　Fos,　tyrosine　hydroxylase　(TH)　or　vasopressin　(VP)　positive　neuron　and　glial　fibrillary　acidic　protein　(GFAP)　positive　astrocytes　in　the　MVZ,　SON　and　PVN　were　observed　by　quadruplicate-labeling　methods　of　WGA-HRP　retrograde　tracing　combined　with　anti-Fos,　TH　(or　VP)　and　GFAP　immunohistochemical　technique.　Fos　positive　neurons　within　the　MVZ,　PVN　and　SON　increased　markedly.　There　were　also　a　large　number　of　GFAP　positive　structures　in　the　brain　and　their　distribution　pattern　was　fundamentally　similar　or　analogous　to　Fos　positive　neurons　in　the　above-mentioned　areas.　The　augmented　GFAP　reactivities　took　on　hypertrophic　cell　bodies,　thicker　and　longer　processes.　Quadruplicate　immunohistochemical　staining　showed　that　a　neuron　could　be　closely　surrounded　by　many　astrocytes　and　they　formed　neuron-astrocytic　complex　(N-ASC).　Fos+/TH+/HRP+/GFAP+　and　Fos+/VP+/HRP+/GFAP+　quadruplicate　labeled　N-ASC　could　be　found　in　the　MVZ,　PVN　and　SON,　respectively.　The　present　results　indicated　that　the　neurons　and　astrocytes　might　be　very　active　following　hyperosmotic　pressure　and　N-ASC　as　a　functional　unit　might　serve　to　modulate　osmotic　pressure.　There　were　reciprocal　osmoregulation　pathways　between　the　MVZ　and　SON　or　PVN　in　the　brain.　(C)　2008　Published　by　Elsevier　Ltd　on　behalf　of　International　Federation　for　Cell　Biology.