To study the established but not well understood phenomenon of improved intratreatment vascular stability during hemofiltration the same 10 stable hemodialysis patients were investigated during one hemodialysis and one hemofiltration treatment. Both treatments were matched in regard to linear fluid withdrawal (3 kg/240 minutes), small molecule removal rate (Curea 120 ml/min). Sodium (140 meq/l) and acetate (35 meq/l) concentration in dialysate and replacement fluid were identical. Outcome measures included mean arterial blood pressure, total peripheral vascular resistance by thermodilution, plasma noradrenaline concentration as index of sympathetic activity and sodium loss per treatment. Blood pressure was maintained during hemofiltration, while total peripheral vascular resistance and plasma noradrenaline concentrations increased. During hemodialysis mean arterial blood pressure fell significantly, total peripheral resistance and plasma noradrenaline concentrations remained unchanged. During both treatment modalities sodium loss was comparable. It is concluded, that the improved hemodynamic stability during hemofiltration is due to a maintained physiologic response to ultrafiltration, which is impaired during hemodialysis. Vascular stability during hemofiltration is not due to sodium retention relative to hemodialysis.