1. During the development and maintenance of one-kidney, one-clip hypertension in rats, there were no significant differences in blood volume or interstitial fluid pressure compared with one-kidney control rats.

2. Rats with one-kidney, one-clip hypertension of 25 days' duration were either unclipped, sham-operated or nephrectomized. Over 24 h the changes in arterial pressure, blood volume and interstitial pressure in the sham and nephrectomized groups were insignificant. In the unclipped group arterial pressure decreased 50 mmHg to normal by 24 h and was associated with increased diuresis and a small decrease in blood volume (9.8%). Unclipped rats with the greatest urine outputs showed the largest decreases in interstitial pressure (r = 0.70, P<0.02).

3. Thus, in the reversal of this hypertension, changes in blood volume played only a partial role; decreased interstitial pressure was secondary to tissue fluid depletion from increased diuresis and normalization of arterial pressure required the presence of the unclipped kidney.

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