The dialysance of acetate and bicarbonate measured using a 1.8 M2 hollow fiber artificial kidney was quantitated both in vivo and in vitro. The diffusional transfers of acetate and bicarbonate were similar as would be expected from their almost identical ionic weights. However, ultrafiltration increased bicarbonate loss and decreased acetate uptake and at a TMP of 300 mm Hg the dialysance of bicarbonate was significantly greater than the dialysance of acetate. Therefore, ultrafiltration reduces the transfer of net base to the patient which impairs the correction of acidosis, and may contribute to clinical problems associated with ultrafiltration during hemodialysis.