Effects of metabolic acidosis were compared between bicarbonate dialysis (BCD) and acetate-free biofiltration (AFB). Three stable dialysis patients (1M, 2F, mean age 30 yrs) were selected for the study because their bicarbonate (BC) pre-dialysis plasma concentration were always under 16 mmol/l while they were on 33 mmol/l-BCD thrice weekly for 12 months. They were switched to a 6 months period of AFB. Pre- and post-dialysis BC plasma concentration, other blood chemical parameters and mass removal (total collection of used dialysate) of urea (U), creatinine (Cr), uric acid (UA), and phosphate (P) were measured during the last week of each period, including 3 dialysis sessions. Mean calorie and protein intake were 29.4 KCal/kg.d and 1.5 g/Kg.d (BCD period) and 38.2 Kcal/Kg.d and 1.5 g/Kg.d (AFB period) respectively. BC plasma concentration (Mean +/- SE, mmol/l) at the pre and post-dialysis in AFB were significantly higher than those in BCD (16.6 +/- 0.7 vs 20.8 +/- 0.6; p less than 0.001, 22.7 +/- 0.8 vs 25.8 +/- 0.8; P less than 0.02). Pre- and post-dialysis U plasma concentration (Mean +/- SE, mmol/l) in AFB were significantly lower than those in BCD (34.3 +/- 2.51 vs 20.8 +/- 0.59, 10.5 +/- 1.32 vs 7.5 +/- 0.92; P less than 0.001). Pre-dialysis P plasma concentration (Mean +/- SE, mmol/l) in AFB was significantly lower than that in BCD (1.85 +/- 0.09 vs 1.50 +/- 0.15; P less than 0.01). Cr, UA and P mass removal in BCD and AFB were not significantly different. However, U mass removal in AFB was significantly lower than that in BCD.(ABSTRACT TRUNCATED AT 250 WORDS)