To determine the rate of acetate transfer across hemodialysis membranes and to investigate the factors which influence acetate dialysance, we performed experiments with Dow-4, Gambro-13.5, and Travenol UF-II artificial kidneys using 14C-acetate. The rate of acetate dialysance was influenced strongly by the rate of fluid flow through the blood path, similar to the dialysance of other low-molecular-weight solutes. The ultrafiltration of fluid from the blood path to dialysate reduced the rate of transfer of acetate from dialysate to blood. The dialysance of acetate was significantly less than that of urea, despite their nearly identical molecular weights. This was not due to the direction of solute transfer. Experiments with a Kaufman-Leonard dialyzer demonstrated that the permeability of Cuprophane membranes to urea was greater than that to acetate was; this may be due to the charged nature of acetate.