From previous work it is known that in subjects with a transplanted kidney treated with cyclosporin A hyperkalaemia may develop even if the glomerular filtration rate is within the normal range or only slightly reduced. The mechanism of this defect was not elucidated so far. In the present work the authors try to study the question whether and how renal potassium excretion by the transplanted kidney is influenced by the intensity of sodium excretion. Focused on renal excretion of potassium, sodium, chlorides, all osmotically active substances, glomerular filtration rate (polyfructosan clearance) and renal plasma flow (paraaminohippuric acid clearance) the authors examined 12 patients after transplantation of the kidney from a cadaverous donor (group A), 11 subjects after unilateral nephrectomy for the purpose of donorship for transplantation (group B) and 27 subjects after nephrectomy on account of a pathological process in one kidney (group C). The glomerular filtration rate in the investigated groups was greater than 1.0 ml/s/1.73 m2. The mean value of the fractional potassium excretion (FEK) in group A was 15.2 (+/- 6.3)%, in group B 18.4 (+/- 6.6)% and in group C 20.1 (+/- 8.6)%. The value of FEK in group A was significantly lower than in group C (p < 0.01). Groups B and C did not differ significantly in the mean value of FEK. Between values of FEK and FENA a significant direct correlation was found (r = 0.621, p < 0.001) in the group of subjects with a single kidney of their own (B + C). On the other hand, this correlation was not found in subjects with a transplanted kidney (A). The achieved results support the idea that in subjects with a transplanted kidney treated with cyclosporin A there are deviations in tubular potassium transport even when its serum level is not elevated. This deviation is manifested by lower FEK values and also by and inadequate response of the distal tubule to an increased sodium supply by increased tubular potassium secretion. The authors assume that when drugs with a potential potassium retaining effect are administered to subjects with a transplanted kidney it is important to check carefully the serum potassium level even when the glomerular filtration rate is within normal limits or only slightly reduced.