A quantitative ultrastructural study was performed to determine the changes in the neurosecretory neurons of the supraoptic (SON) and circularis (NC) nuclei following 4-24 h of water deprivation (WD) and subsequent rehydration (12 and 24 h). In both nuclei, the amount of direct soma-somatic contact increased throughout WD, apparently by retraction of fine glial processes from between the cells. Rehydration reversed these changes. The number of smaller (less than 1600 A) neurosecretory granules (NSG's) decreased in both nuclei at 4 h of WD but returned to control levels by 24 h of WD and remained so during rehydration. Larger (less than 1600 A) NSG's decreased in number at 4 h of WD in SON and then returned to control levels by 24 h of WD and remained the same throughout rehydration. In NC, these NSG's did not change in number with WD, but significantly increased between 12 and 24 h of rehydration. No cells with dilated rough endoplasmic reticulum were seen in NC during this study. In SON, however, the percentage of such cells increased at 4 and 12 h of dehydration only to decrease to control levels at 24 h of dehydration and throughout rehydration. Lysosomes decreased at 4 h of dehydration in SON and returned to control levels thereafter. In NC, lysosomes tended to decrease with dehydration and increase with rehydration. These findings indicate that detectable morphological changes take place in the course of alterations in hydration state that are well within the physiological range.