1. The sodium and water excretion rates of rabbit kidneys were studied when isolated and perfused at known pressure with blood from another normal anaesthetized rabbit. Studies at several different perfusion pressures confirmed that a small rise in perfusion pressure caused a large rise in sodium excretion and that the potential sodium-excreting ability of the isolated kidney was high. The curve obtained could be closely fitted by a quadratic equation which allowed an estimate to be made of the blood pressure below which no urine is formed, i.e. the ‘theoretical perfusion pressure threshold’. For normal kidneys this was 55·4 mmHg.
2. A group of rabbits had a silver clip applied to the left renal artery and, 3–6 weeks later, the eight most hypertensive animals were selected to provide their kidneys for perfusion. Both kidneys were perfused simultaneously. The clip on the left renal artery was removed immediately before perfusion and the cannula placed distal to the stenosis in the post-stenotic dilatation. The function curves of these kidneys were compared with the curves obtained from normal kidneys.
3. The untouched kidney contralateral to the clip was found to require a significantly higher perfusion pressure (71·7 mmHg) for it to achieve a given sodium excretion rate and, surprisingly, the clipped kidney showed a similar functional change (76·4 mmHg). In other words the positions of both function curves were shifted though their slopes were not much changed.
4. Both kidneys in single-clip-hypertension appear to adapt or reset their sodium excretory behaviour. The resetting in the untouched kidney allows hypertension to be sustained without undue sodium loss. Aldosterone probably contributes little to the resetting. We infer, indirectly, that the normal kidney may, to a significant extent, restrain sodium excretion by virtue of its sympathetic innervation. We also opine that the kidney cannot be assigned fixed intrinsic functional properties on which a renal sodium-handling theory of long-term blood pressure regulation can be firmly based.