The evaluation of free water transport is a tool for assessment of aquaporin function in peritoneal dialysis patients. Sodium dip and the estimation of sieving coefficient for sodium may be used for the quantification of free water fraction in the ultrafiltration flow from blood to the peritoneal cavity. The results of clinical studies show that this fraction is about 0.4, and decreases with peritoneal dwell time. The fraction also varies in different peritoneal equilibration test groups, being the lowest in high transporters.