Neurohypophyses from rats dehydrated and rehydrated for different periods of time have been analyzed by morphometry of electron micrographs and their oxytocin and vasopressin contents measured by radioimmunoassays. The time course of disappearance and reappearance of neurosecretory granules (NSG) parallels that of the hormone content of the neurohypophysis. Depletion of NSG during dehydration first occurs in the nerve endings and only later in the swellings. No change of the volumetric density of the microvesicles was detected but the time course of changes in the volumetric density of the endocytotic vacuoles varies according to the secretory state of the tissue. There was a significant increase in the volume of the gland 3 days after the beginning of the rehydration. This increase was associated with an increase in the volumetric density of the pituicytes. No change of the total number of endings, swellings and axons was observed. This demonstrates a hypertrophy of the pituicytes.