Corticotropin releasing factor and vasopressin were measured in major brain regions including the neurohypophysis in spontaneously hypertensive rats (SHR) and normotensive VVistar-Kyoto rats (WKY) during development of hypertension. The highest concentration of corticotropin releasing factor was found hi the hypothalamus in both strains. Corticotropin releasing factor was decreased in most major brain regions of SHR. In the hypothalamus, corticotropin releasing factor was lower in 3-and 6-week-old SHR than in age-matched WKY (p< 0.01), but was similar at 12 and 24 weeks of age. The content of corticotropin releasing factor did not differ in the neurohypophysis in 3-week-old rats but began to decrease at 6 weeks of age (p< 0.01) and continued to decrease during the development of hypertension (p< 0.01). Brain vasopressin concentration did not differ between SHR and WKY except in the hypothalamus. The level of hypothalamic vasopressin was consistently lower in SHR than in WKY (p< 0.01). These peptides are thought to be associated with autonomic nervous regulation, and our results may further strengthen the possibility that the deficit of the peptides may be involved in the development of spontaneous hypertension.