The aim of the study was the search for the correlation between the degree of impairment renal function (measured by creatinine clearance) and plasma concentration of atrial natriuretic peptide (ANP) and urinary fractional sodium excretion (FENa) in patients with chronic renal diseases. Forty seven patients were studied: 10 with diminished renal reserve (group I), 10 with renal insufficiency (group II), 27 with renal failure (group III) and 10 chronically haemodialysed before dialysis (group IV). Control group consisted of 27 healthy persons. All patients and controls were on the diet containing 100-120 mmol sodium daily. Plasma ANP levels were significantly higher in all groups of patients (I--16.5 +/- 5.7; II--40.7 +/- 18.6; III--86.2 +/- 49.9; IV--196.1 +/- 51.3 pmol/l, respectively) than in controls (10.8 +/- 6.0 pmol/l). A significant correlation (r = 0.85; p less than 0.01) between plasma ANP concentration and FENa was found when the patients from all groups were pooled together. The results confirm the important role of ANP in the adaptation of reduced kidney mass to the excretion of sodium load.