SummaryWith a view to determining the routes of absorption in hydronephrosis dyes have been introduced into thc renal plvis in amounts well below the total pelvic capacity.Two routes of absorption have been noted, i.e., lynphatic and tubular. At the outset of complete ureteral obstruction there ensures for the first two or three days a purely lymphatic absorption from the walls of the renal pelvis and ureter. After about the third day tubular absorption commences and continues more actively than the lymphatic.Even a short period of “back pressure” favours tubular absorption. The convoluted tubules of the peripheral glomeruli are the first to take on this function, and as pressure atrophy progressively supervenes the subjacent layers continue the process of reabsorption.In thus absorbing dye media, the cells of the convoluted tubules may be demonstrating a slightly altered normal function. If so there action would tend to favour the “filtration and reabsorption”. theory of renal secretion.The impression is obtained that “Pyelvenous backflow” is probably due to trauma, and cannot accordingly be regarded as a usual factor in the mechanism of hyd