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Department of Physiology, School of Medicine, University of California, San Francisco, California 94143, U.S.A.

1. Experiments were conducted in isolated perfused rat kidneys to determine the effect of raising perfusate albumin concentration on renin release.

2. Raising albumin concentration in the perfusion fluid from 20 g/l. to 60 g/l. (high albumin concentration) increased renin release and renal perfusate flow rate. The effect was reversible.

3. Ureteral occlusion did not prevent the rise in renin release and renal perfusate flow induced by high albumin concentration.

4. Propranolol (0·28 mM) did not block the renin release stimulated by high albumin concentration, but it inhibited the release stimulated by isoprenaline (2·43 µM).

5. Clonidine (10 µM) and oxymetazoline (10 µM) constricted the renal vasculature and stimulated renin release during high perfusate albumin concentration providing perfusion pressure was kept constant.

6. Low renal perfusion pressure (50 mmHg) and isoprenaline (2·43 µM) stimulated renin release in perfusion experiments with both 20 and 60 g/l., but the rate of renin release was substantially greater with 60 g/l.

7. On the other hand, perfusion fluid deprived of calcium induced a greater increase in renin release in kidneys perfused with 20 g/l. than in those with 60 g/l.

8. We conclude that high albumin concentration stimulates renin release in isolated perfused rat kidneys by a mechanism which does not involve the renal nerve, direct renal vasodilation or sodium excretion. High albumin concentration may increase the sensitivity of the kidney to acute stimulation by a mechanism involving calcium.