1. In 12 week-old stroke-prone spontaneously hypertensive (SPSH) rats and in normotensive Wistar-Kyoto (WKY) rats plasma concentration of [arginine]vasopressin (AVP) and the AVP content in various brain areas were measured by radioimmunoassay.
2. In hydrated SPSH rats plasma AVP was lower than in WKY rats. In the hypothalamus and in the brain stem, but not in the pituitary, the content of AVP was reduced.
3. After a dehydration period of 48 h plasma AVP rose similarly in both SPSH and WKY rats. However, in the pituitary the AVP content was significantly lower in SPSH than in WKY rats. In the hypothalamus and in the brain stem, AVP content was not significantly influenced by dehydration.
4. It is concluded that in the hydrated stage the secretion of AVP and its synthesis in the hypothalamus are reduced in SPSH rats. However, the SPSH rats still respond satisfactorily to the strong stimulus of severe dehydration.