The role of potassium in regulating plasma aldosterone has been studied in 7 patients with end-stage renal disease undergoing hemodialysis. Plasma renin activity (PRA), plasma aldosterone, cortisol, sodium and potassium, and total potassium loss were measured before and after two sessions of dialysis in each patient. The only difference between the two sessions was the potassium content of the dialysate (4 mmol/l versus l mmol/l). Plasma potassium, potassium pool and plasma aldosterone decreased significantly when the patients were dialyzed with a l mmol/l K bath. In contrast, the same parameters remained unchanged when the patients were dialyzed with a 4 mmol/l K bath. Irrespective of the dialysate used, there were no differences either in changes in body weight and PRA, or in plasma sodium and cortisol levels, before and after dialysis. In conclusion, in patients with end-stage renal disease, potassium predominates over PRA and volume depletion in the regulation of plasma aldosterone.