1. Plasma volume, packed cell volume (PCV), blood volume, extracellular fluid volume (ECFV) and Evans blue disappearance rate were measured in conscious spontaneously hypertensive rats and in weight-matched Wistar normotensive rats.
2. Over the weight range studied (250-350 g), plasma and blood volumes were significantly lower in the spontaneously hypertensive rat. Extracellular fluid volumes were similar in the two groups. PCV arid Evans blue disappearance rates were significantly higher in the spontaneously hypertensive rat.
3. Negative correlations were obtained between plasma volume and mean arterial pressure and between the plasma/interstitial fluid volume ratio and mean arterial pressure.
4. the normal extracellular fluid volume and the lack of correlation with mean arterial pressure excludes volume expansion as a pressor mechanism during the established phase of hypertension in the spontaneously hypertensive rat.