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Body fluid volumes were measured in conscious spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats from four to 16 weeks of age.Plasma volume was elevated in four-week-old SHR, similar at five weeks, and decreased at all other ages compared with WKY controls. Blood volume showed a similar pattern. Extracellular fluid volume was found not to be different between the two strains in age-matched animals, except at four and five weeks where the SHR exhibited lower values than the WKY. The plasma/interstitial fluid volume ratio was significantly elevated at four weeks, but decreased from six weeks on in SHR compared with WKY controls. Sodium space tended to be increased in young SHR (four to six weeks), but was similar at eight and 16 weeks for the two strains.These results indicate that: (1) intravascular volume expansion may be involved in the development of hypertension in SHR in the very early stages; (2) intravascular volume changes do not appear to be related to overall volume expansion or contraction.