1. Measurements of plasma volume, haematocrit, extracellular fluid volume, blood pressure, venous pressure and interstitial tissue pressure were made in rats 4 days after unilateral nephrectomy. Measurements were repeated either 4 days after subsequent removal of the remaining kidney, or 4 days after anastomosis of the remaining ureter with the inferior vena cava. Extracellular fluid volume was expanded by giving 0.5% saline by mouth: in one series blood volume was expanded by injections of blood.

2. Plasma volume (PV) rose more after bilateral nephrectomy (BN), extracellular fluid volume (ECFV) more after unilateral nephrectomy and ureterocaval anastomosis (UNUCA). The PV/ECFV ratio was significantly higher after BN.

3. Blood pressure and venous pressure rose after BN but not after UNUCA. Interstitial tissue pressure (TP) rose more after BN in spite of greater expansion of interstitial fluid volume (IFV = ECFV-PV) after UNUCA. The ratio ΔIFV/ΔTP was several times less after BN than after UNUCA.

4. Interstitial space compliance was estimated by measuring changes in IFV and TP 10 min after a saline infusion (compliance = ΔIFV/ΔTP). After UNUCA there was little change from values obtained after unilateral nephrectomy alone; following BN compliance fell severalfold.

5. It is suggested that changes in compliance of the interstitial space may be brought about by a substance secreted by the kidney and represent a hitherto undetected mechanism by which plasma volume is maintained constant under different conditions of hydration. Changes in interstitial space compliance may also play a part in the aetiology of renoprival hypertension by raising tissue pressure, and thus venous pressure and cardiac output.

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