Monitoring peritoneal permeability is indispensable to adjust the dialysis procedure to the characteristics of each individual patient and to detect and prevent the complications of peritoneal dialysis. Several monitoring techniques can be used, the simplest one being the most inaccurate. The authors consider that in routine practice the first alterations can be detected merely by measuring ultrafiltration. The next step is a single glucose assay after a 4-hour long contact. Evaluating the speed of diffusion of glucose and urea by means of four assays in two hours is an excellent compromise: the equilibration curves thus obtained can be summarized as the time required for the percentage of glucose absorption to be identical with the percentage of dialysate saturation in urea (APEX time).