Large surface area, high mass transfer dialysers have recently come into widespread use, and it has been shown that they promote the loss of large amounts of bicarbonate when acetate is used in the dialysate. The present study was designed to confirm these metabolic consequences and quantitatively to compare two commonly used dialysers in this respect. Simultaneous measurement of arterial and venous blood gases has shown that there is a significant reduction in bicarbonate, carbon dioxide tension, and pH of blood returning to the patient from the dialyser. These changes were greatest with the 2,5-m2 hollow-fibre dialyser, and least with the 1,4-m2 coil dialyser. In the chronic dialysis patient in a steady state, these effects may be inconsequential but, in an acutely ill patient, the combination of a dialysate containing acetate and a high-efficiency dialyser may be extremely hazardous. A return to the use of bicarbonate as the source of base would avoid such hazards and would promote the more physiological correction of the metabolic acidosis of renal failure.