Two groups of subjects (uraemic and control) were studied. The uraemic group consisted of 30 patients treated by HD (haemodialysis). The mean age was x +/- SD-35.4 +/- 8.4 years, duration of haemodialysis treatment 42.8 +/- 12.1 months, cuprophan dialyzers and acetate containing solution--35 mEq/l--were used, duration of HD-4 hours 3 times weekly, predialysis serum creatinine was 1060 +/- 218 mumol/l (12.8 +/- 2.5 mg%). The control group comprised 23 healthy subjects (mean age 33.0 +/- 8.0 years, serum creatinine level 88.4 +/- 14 mumol/l (1.0 +/- 0.16 mg%). In all examined subjects the following experimental protocol was used. Blood pressure (BP) was determined at about 8 a.m. after an overnight rest. Then blood samples were withdrawn for estimation of ANP, AVP, sodium and potassium, protein, osmolality and creatinine concentrations. Between 8 and 12 a.m. all examined uraemic subjects were dialysed. After each hour of dialysis BP was measured and blood samples were taken. ANP (Peninsula Lab. Kids.) and AVP (DRG) were measured using RIA method, and other biochemical parameters using routine methods. Plasma creatinine and plasma ANP levels significantly decreased, but AVP significantly increased after HD. CONCLUSIONS 1. In all uraemic subjects, plasma ANP and AVP levels were significantly higher than in control subjects; 2. During haemodialysis with ultrafiltration a significant increase AVP level and decrease ANP level was observed; 3. A significant correlation between ANP concentration and blood pressure may suggest participation of above mentioned hormone in pathogenesis of hypertension in patients with uraemia; 4. It's possible, in pathogenesis plasma AVP increase takes part plasma ANP decrease, too.