It has been known that magnocellular neuroendocrine cells (MNCs) of mammalian hypothalamus show structural plasticity in response to chronic osmotic stimulation. In this study, we investigated the relationships among plasma osmolarity and several structural changes such as alterations of soma size, juxtaposition, and synapses of the supraoptic nucleus (SON) in the rat hypothalamus during dehydration and rehydration. Male rats were osmotically stimulated by supplying with 2% NaCl solution instead of tap water for 10 days, and then they were rehydrated with tap water. Plasma osmolarity was gradually elevated with progress of salt loading and returned to control level on the seventh day of rehydration. Both the percentage of membrane contact (juxtaposition) and the soma size of MNCs were increased in response to the rise of plasma osmolarity, and decreased to control level on the seventh day of rehydration. The number of synapses including both single synapses and multiple synapses per 100 microns soma membrane was lower than control on the fifth day of dehydration, but it was not different from controls on the tenth day of dehydration, and on the seventh and fourteenth day of rehydration. The total number of synapses per 100 microns soma membrane, the synaptic density, was maintained relatively constant, although soma size was progressively changed during dehydration or rehydration. This synaptic reorganization seems to be mainly regulated by synaptic sprouting during dehydration and by degradation of synapses during rehydration.(ABSTRACT TRUNCATED AT 250 WORDS)