Eight stable patients on maintenance hemodialysis were studied while undergoing (a) acetate hemodialysis with a sequential dialysate sodium concentration from 147 to 137 mEq/L (SNa-HDA) and (b) bicarbonate hemodialysis with a constant dialysate sodium concentration of 140 mEq/L (HDB). Circulatory behavior was observed during both of these methods, and both were found to allow a high volume removal. However, as a consequence of the high sodium load during SNa-HDA, volume was shifted from the extra- to the intravascular space. This stabilizing effect on the circulation disappeared with the sequential decrease of dialysate sodium concentration (despite a constant plasma sodium concentration (despite a constant plasma sodium concentration of approximately 140 mEq/L), which was concomitant with a significant decline of the mean arterial blood pressure and an inadequate compensation of the metabolic acidosis. In contrast, a better circulatory response to comparable volume removal was found during HDB, expressed by a stable mean arterial blood pressure in the presence of well-balanced arterial acid-base values.