Objective To investigate the response of neuron-astrocytic complex (N-ASC) in the supraoptic nucleus (SON) and solitary tracto nucleus (NTS) to hypertonic stimuli (9%NaCl, 5.5ml/kg) injecting into the trail vein of rats. Methods Male SD rats were randomly divided into two groups and were used for light microscopy(EM>n/EM>=40) or electromicroscopy (EM>n/EM>=10) study respectively. This research contained two parts. The first part, we used double immunohistochemistry to examine the spatial relationship among glial fibrillary acidic protein (GFAP, expressed in astrocytes), Fos (label activated neurons) and oxytocin (OXT, expressed in SON neurons) or tyrosine hydroxylase (TH, expressed in NTS neurons) positive cellular structures at 15, 45, 90 and 180 minutes after stimulation. In the second part, we obserberd the spatial arrangement between connexin 43 (Cx43) and connexin 32 (Cx32)immunopositive elements at ultracellular level in SON; samples were prepared 90 minutes after stimulus. Results Three types N-ASC were observed in SON and NTS: The first type was seen between GFAP positive astrocytes and OXT or TH positive neurons, but these neurons were Fos negative. The second type was located between GFAP positive astrocytes and Fos positive neuronal nucelei, however these neuronal perikarya was OXT or TH negative. The third type exhibited triple labeled feature, i.e. GFAP positive astrocytes, Fos positive neuronal nucelei and OXT or TH positive neuronal perikarya. The number of N-ASC increased significantly after hyperosmotic stimuli. Two types of untrastructures between astrocytes and neurons of N-ASC within SON were observed. One type was Cx43 hemichannels (Cx43 positive astrocytic processes), another was the heterotypic gap junctions (HGJ), which consisted of Cx43 positive astrocytic processes and Cx32 positive dendrites. The number of Cx43 hemichannels and HGJ increased significantly after hypertonic stimulus. Conclusion N-ASC in SON and NTS, as a functional unit, might be involved in modulating osmotic pressure.