Tolerance of peritoneal dialysis is, in a part, dependent on intraperitoneal dialysate volume. Measurement of intraperitoneal pressure (IPP) in cm of water is easy to perform especially with the twin bag Y set (Baxter). Today we use IPP for following surgical catheter implantation (delaying, starting and progressing with peritoneal dialysis) and for optimizing ultrafiltration and purification. Efficiency of peritoneal dialysis is dependent on adequate ultrafiltration (UF) and on adequate purification (solute clearances). These two goals seem apparently conflicting in terms of duration of dwells: short dwell time enhances UF capacity and conversely long dwell time enhances solute clearance. Peritoneal equilibration test (PET) allows an approach to the ultrafiltration time: the point at which the overtime dialysate urea saturation and glucose desaturation curves cross, called APEX time. PET allows also an approach of the purification time: the point at which dialysate (D) to plasma (P) concentration ratios over time are high. Because the value of phosphate as uremic factor of morbidity, we have chosen the time for D/P phosphate equal to 0.6 as a purification phosphate dwell time (PPT). A total of 17 patients were studied, over a five-year period allowing 142 determinations. APEX times (range 18 to 71 min) and PPT (range 105 to 238 min) were spread over a wide distribution. PPT and APEX times were significantly shorter in children younger than 3 years of age than in children older than 10 years of age. PPT were nearly four times longer than APEX times. The knowledge of these conflicting ultrafiltration and purification times should help, in our view, in the individual choice of the PD modality: if UF is the major goal, short dwell times should be used (automatic PD); if purification is the major goal, long dwell times should be used (CAPD); if both are the target goal, tidal PD should be discussed.