The effect of ultrafiltration on blood pressure was compared in six patients with problems of overhydration during single-pass dialysis, ultrafiltration only (the fluid bypassing the dialyser) and recirculation dialysis (using the RP 6 dialyser and the 75 l Rodial system). Adequate ultrafiltration during single-pass dialysis was hampered by symptomatic hypotension. However, during recirculation dialysis as well as during ultrafiltration without dialysis much more rapid ultrafiltration was tolerated without a fall in blood pressure. A considerable fall in plasma osmolality occurred with single-pass dialysis, whereas only a small decrease occurred with recirculation dialysis and no change with ultrafiltration only. We conclude that a rapid fall in plasma osmolality interferes with blood pressure regulation and that ultrafiltration is far better tolerated provided that osmotic shifts are avoided or minimised.