1. The effect of removal of the clip on blood pressure, plasma renin activity and the excretion of water, sodium and potassium was studied in renal hypertensive rats. Hypertension was induced by application of a clip with an internal diameter of 0·20 mm, which was removed after 1, 2 or 3 weeks, and by using a clip with an internal diameter of 0·25 mm, which was removed after 2 weeks. The contralateral kidney remained undisturbed.
2. Blood pressure was almost normal 24 h after the removal of the clip; 70–90% of the total decrease in blood pressure occurred within 2–5 h.
3. The increased plasma renin activity, which was observed 2 and 3 weeks after application of a 0·20 mm clip, had returned to control values 24 h after removal of the clip. One week after application of a 0·20 mm clip, and 2 weeks after a 0·25 mm clip, plasma renin activity did not differ significantly from control values, before as well as after unclipping.
4. No significant differences were found between unclipped and sham-operated renal hypertensive rats, nor between the latter and the sham-operated normotensive control rats for water, sodium and potassium excretion, and for change in body weight, during the 24 h after the removal of the clip.
5. It is concluded that urinary loss of water and/or sodium does not play an important role in the acute decrease of blood pressure which occurs after the removal of a renal artery clip in one-clip, two-kidney hypertensive rats. A decrease in peripheral plasma renin activity can only partly explain the reversal of the hypertension.