To evaluate the effect of irrigant flow (IF) on intrarenal pressure (IP) and on renal parenchyma during and after ureteroscopy (URS) by using an “ex vivo” porcine urinary track model. We four times practiced the same experiment using three different procedures of (URS): flexible ureteroscopy (FURS), flexible ureteroscopy with ureteroscopic access sheath (FURS+UAS) and rigid ureteroscopy (RURS). Each time repeated twice, we used six pairs of porcine kidney, three males and three females, all from the same pigsty. For each procedure, we used one male and one female pair. And for each pair, we took one kidney as the treatment case the other as the control case. With two different irrigant flows (100ml/min and 200ml/min), the IP was recorded, then we used a SPSS 17.0 statistical analysis software to calculate the P value. Morphological changes of the kidney were photographed and samples were taken after dissection of the kidneys for histological examination (H-E). During the experiment, we morphologically observed three main changes: (1) the volume of the kidney increases and the shape changes within five minutes (all groups). (2)The perforation of the kidney parenchyma is leading to the passage of water into the subcapsular space within six and eight minutes (RURS group and FURS group respectively). (3) A high subcapsular water retention causing a capsular burst after eight minutes (RURS group). For IP, with IF1 (100 ml/min), we measured IP 21.5±3.1cmH2O, 68.3±3.5cmH2O and 137.0±6.6cmH2O for FURS+UAS, FURS and RURS respectively; while IF2 (200ml/min), IP was 25.3±2.5cmH2O, 73.1±4.5cmH2O and 144.9±7.0cmH2O for FURS+UAS, FURS and RURS respectively. The SPSS 17.0 software calculated P < 0.05, meaning that the difference was significant between the studied groups. In H-E, we noticed an increasing glomerulus number (all groups) plus a small amount of inflammatory cell fluid and expanding small blood vessels (FURS group), and compensatory expansion of glomerulus with small amount of focal bleeding (RURS group). If is undeniable that RIRS techniques are all efficient for stones extraction and their impacts on the upper urinary tract often lead to well controlled postoperative complications, it is henceforth more than necessary to emphasize that some of these techniques remain dangerous and can lead, when they are not properly used, to irreversible complications. Even not giving full explanation of post ureteroscopy complications, these results should be at least taken as a serious warning. Therefore, as well as RURS still the most accessible and cheapest technique, we strongly advice surgeons to always perfuse a very slow irrigant flow during the procedure to avoid irreversible complications. Even more, we suggest using the FURS+UAS technique that provides well irrigant flow while maintaining IP below threshold for upper tract backflow, at the expense of simple FURS and RURS