The arrangement of renal tubules and blood vessels is so unique that many details of structure still remain unknown, and this may in part account for our continued ignorance of details of function. Perhaps the following characteristic of renal structure may be new to many; only rarely and vaguely is it found mentioned in the literature, and never has it been subjected to the experimental study that its significance merits. There occurs, under conditions of moderate intrapelvic pressure, a back flow of pelvic contents into the renal veins. The degree of back pressure producing it is generally less than renal secretory pressure and, the communication once established, a lower pressure than the initial one keeps it going. Curiously, relief of pressure is rarely, if ever, followed by hemorrhage. "Pelvic reabsorption" is a descriptive term, but too general, in that the route might be mucosal, tubular, lymphatic or what not. Besides,