A condition of metabolic alkalosis has been generally observed in patients undergoing regular biofiltration (BF). The aim of this study was to assess buffer kinetics during a dialysis session in five patients regularly treated by BF for 10 months (dialyzer Biospal 3000S, Qb 301; Qd 545; UF 36.76 ml/min; dialysate CH3 COO 38 mEq/L; replacement fluid 1000 ml/h with HCO3 100 mEq/L solution). As usually happens using dialyzers with high surface area and permeability, large HCO3 losses (756 +/- 112 mEq) and CH3COO uptakes (677 +/- 152 mEq) were observed. Acetate plasma levels rose to 10.4 mEq/L, a value potentially dangerous to the cardiovascular system. The dialytic buffer gain (acetate uptake + HCO3 administered - HCO3 loss) was high (230 +/- 137 mEq/dialysis) and excessive for patients' needs. It is therefore very important to reduce either the acetate concentration in dialysate or the amount of reinfused bicarbonate. In view of the plasma acetate levels it is preferable to use a dialysis solution containing less acetate.