Reflux of urine up the ureters provides a way in which infection can be conveyed to the kidney from an infected bladder. Once infected urine reaches the pelvi-calyceal system it may invade the parenchyma by one of several routes. In the first place it may invade the parenchyma by reflux up the ducts of Bellini, the so-called intrarenal reflux or calicotubular reflux. In the rat the kidney may be invaded in the fornicial region close to the insertion of the calyceal wall and it is likely that this also occurs in the newborn. Invasion via the veins — pyelovenous route — is also considered possible but is not so well documented as the other routes. Once infection has been established in the kidney remarkable degrees of tubular destruction take place and the acute inflammatory focus heals with considerable scar formation.