1. Isolated kidneys taken from spontaneously hypertensive rats (SHR) and normotensive Wistar–Kyoto rats (WKY) were perfused over a range of perfusion pressures.
2. Lithium clearance was used as an index of proximal tubule sodium handling.
3. When the perfusate contained an oncotic agent (albumin, 6.7 g/dl) the SHR kidneys performed differently from the WKY kidneys with a reduction in inulin clearance, sodium excretion, fractional sodium excretion and fractional lithium excretion [at 105 mmHg (14 kPa) perfusion pressure, SHR 6.0 ± 1.1% vs WKY 12.6 ± 2.4% (mean ± sem); at 150 mmHg (20 kPa), SHR 17.1 ± 1.6% vs WKY 27.0 ± 2.3%]. Calculated indices of distal tubular function showed no major differences between SHR and WKY.
4. When kidneys were perfused without oncotic agent in the perfusate the differences between SHR and WKY in tubular handling of sodium and lithium were largely abolished.
5. These findings are consistent with the hypothesis that increased sodium reabsorption occurs in the proximal tubules of the kidneys of SHR and suggest that this is an intrinsic property of the kidney, not immediately dependent on neural or humoral factors. Increased sodium reabsorption in the proximal tubule may contribute significantly to the existence of hypertension in the SHR.