1. To determine the part played by changes in interstitial tissue compliance in the mechanism of experimental renal hypertension, pressure and volume measurements were made in rats 60 days after partial constriction of one renal artery with a clip and removal of the opposite kidney. Tissue pressure and venous pressure were found to be significantly higher in rats which developed systolic arterial blood pressure of 140 mmHg or above than in loosely clipped controls with normal blood pressure. Plasma volume (PV) was also increased in the hypertensive animals, but there was no difference in interstitial fluid volume (IFV); the PV/IFV ratio was significantly higher in the hypertensive rats.
2. Removal of the clip in the hypertensive rats restored blood pressure to normal in 1 h; this was associated with a fall in venous pressure and plasma volume. Tissue pressure fell despite a rise in interstitial fluid volume; the PV/IFV ratio fell.
3. It is suggested that renal artery constriction, possibly by a humoral mechanism, causes a fall in interstitial space compliance and that this, by causing changes in body fluid distribution, plays a part in the mechanism of experimental renal hypertension. This mechanism may have a physiological role in the maintenance of adequate plasma volume during water shortage.