1. Exchangeable sodium (NaE) was measured serially in rats given a sodium-free diet to eat with sodium chloride solution (85 mmol/l) containing 22Na to drink.
2. After 15 days, nine rats had a left renal artery clip applied; nine had a sham operation.
3. There was no significant difference in NaE between the two groups during the 6 week period after clipping, at the end of which blood pressure was 189 ± 8 mmHg (mean ± sem) in the clipped group and 150 ± 2 mmHg in the sham-operated group (P < 0·001).
4. When the clips were removed blood pressure in the hypertensive group fell to 144 ± 4 mmHg.
5. On the first day after removal of the clip NaE was significantly lower in the clipped group than in the sham-operated group, but there were no significant differences thereafter.
6. Total body sodium (TBNa), measured at death, was consistently higher than NaE by a mean of 1·25 ± 0·08 mmol, in hypertensive and control rats alike.
7. We conclude that changes in sodium balance are not a necessary accompaniment of the development of hypertension in this two-kidney one-clip rat model.