An accurate evaluation of water content in the body of hemodialyzed patients seems to be an important problem in chronic dialysotherapy. Acute intradialytic hypotension observed in 20-33% of hemodialysis patients is a very common complication of this kind of renal replacement therapy. The study was performed in 40 uremic patients, treated with hemodialysis at the Nephrology Clinic of the University Hospital in Cracow. In every patient, 3 model dialysis sessions were carried out. Total fluid removal was the same during every hemodialysis. The first model hemodialysis (HD1) was performed with constant dialysate sodium concentration (140 mmol/L), the second (HD2) with linear and the third (HD3) with expotential decrease of dialysate sodium concentration (from 144 to 136 mmol/L). Every hemodialysis was also monitored continuously with Crit-Line 2 R system (In-Line Diagnostics, Riverdale, UT). Before and after the first model hemodialysis (HD1), ultrasound examination of abdominal cavity was performed. The measurement of inferior vena cava diameter (mm), circumference (mm), area (mm2), at hepatic veins orifice-level, on expiration was performed. The fluid removal during the first model hemodialysis resulted in significant reduction of the vena cava inferior diameter, circumference and area, measured with ultrasound on expiration. The statistically significant lower fall of blood volume after the first and second hour of the second model hemodialysis session (HD2) was observed when compared to the first hemodialysis (HD1) - p<0.05 was observed. The statistically significant lower frequency of hypotension during the second hemo-dialysis session (HD2) as compared to HD1 (chi2=5.25 p<0.05). Differences among HD1 and HD3 and HD2 and HD3 did not reach statistical significance. The monitoring of hemodialysis with the Crit-Line instrument permits for optimalization of dry weight of dialyzed patients and allows reaching higher ultrafiltration rates during dialysis without hypotensive episodes. The changes in the blood volume, approximately 5% per one hour of dialysis session are an optimal value for these patients.