1. Plasma concentration of arginine vasopressin, plasma osmolality and packed cell volume were measured in stroke-prone spontaneously hypertensive rats (SHRSP) and in normotensive Wistar-Kyoto (WKY) rats at different ages.

2. In young and in adolescent SHRSP, at 6, 9 and 12 weeks of age, plasma concentration of vasopressin was diminished as compared with age-matched WKY rats (P < 0.01), whereas, in 18-week-old rats, the difference was not significant (P > 0.05). In contrast, in 24-week-old rats, plasma vasopressin was elevated as compared with WKY rats of the same age (P < 0.01).

3. In none of the age groups did plasma osmolality differ between the two strains of rats, but in all groups packed cell volume in SHRSP was higher than in WKY rats.

4. During a 48 h period of dehydration, plasma vasopressin concentrations increased similarly in SHRSP and in WKY rats of 6 and 12 weeks of age respectively.

5. It is concluded that vasopressin does not contribute to the development of high blood pressure in SHRSP rats. The reduced plasma concentration of vasopressin may account for the decreased plasma and blood volume and the slightly elevated plasma sodium concentration observed in young SHRSP rats.

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