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Morphological plasticity of astrocytes has been hypothesized as vital to ongoing plasticity in the adult central nervous system. However, it has yet to be demonstrated directly that astrocytes are capable of gross morphological changes under non-pathological circumstances in the adult nervous system. In our studies, we have investigated this issue in the glial limitans associated with a well-known model of neuronal plasticity, the hypothalamic supraoptic nucleus. The activity of the neuroendocrine cells comprising this region was stimulated in rats by substitution of 2% saline for drinking water for 2 or 9 days. Light and electron microscopic morphometry revealed that by 9 days of stimulation a significant decrease in glial limitans thickness and a significant overall reorientation of glial limitans astrocytes occurred. Further, an electron microscopic investigation revealed the amount of basal lamina in the glial limitans also decreased with dehydration. In animals allowed to rehydrate for 9 days these changes were entirely reversed. We conclude that astrocytes in vivo are indeed capable of reversible gross morphological changes over a relatively short time. We hypothesize that the reorientation of these glial limitans astrocytes accounts, at least in part, for previously reported structural remodelling in the nucleus, as well as contributes to the newly observed reduction in glial limitans thickness found in our studies. Functional consequences of this reduction could include facilitating the movement of diffusible substances between the nucleus and underlying cerebrospinal fluid, or enabling osmotic signals in the cerebrospinal fluid to reach the neurosecretory cells. As basal lamina has been suggested to play a role in the maintenance of cell polarity and in the prevention of regulated cell death, we suggest a decrease in this extracellular matrix may be a mechanism involved in our observed changes in astrocyte morphology and may additionally play a role in maintaining astrocyte viability.