Printed in Canada. All rights reserved.

During hypertonic peritoneal dialysis (PD) dwells,an initial reduction in the dialysate sodium con-centration regularly occurs with dwell time, owing to sodium sieving. Minimum dialysate Na concentration is attained after approximately 80 – 120 minutes; af-ter which, the dialysate Na again approaches the plasma Na concentration. This phenomenon is most likely a result of osmotic water transport through endothelial water channels aquaporin-1 (AQP-1), which reject solutes and thereby cause dilution of the dialysate over the first 2 hours of the dwell (1–5). In severe ultrafiltration failure (UFF), an apparent lack of sodium sieving is a frequent occurrence (6). Lack of sodium sieving has usually been taken as evidence of reduced water transport through aquaporins, con-ceivably due to malfunction of those molecular wate