Data are presented showing the locus of concentration of dyes and Fe salts within the lumen of the distal convolution of the renal tubule of 3 classes of vertebrate kidneys. The site of water reabsorption is indicated by the presence of these substances in highly concentrated form only in the lumen of this convolution. Special methods were devised to show convincingly the locus of this concentration, and the precise conditions under which this concentration occurs or may be induced are set forth. Kidneys whose visible glomeruli are inactive, permitting the lumina of tubules connected with them to become filled with concentrated substance, are quickly washed free of it by reactivating these glomeruli. The significance of the rapid release of fluid from the glomerulus is suggested and its effect on the relative patency of the lumen of the tubule is shown. The rate of excretion by the tubule is relatively slow and uniform with or without continuous glomerular activity as shown by the careful inactivation of a single glomerulus in a highly functional kidney. New methods are described for inactivating a given glomerulus as well as in part for studying segmentally, without sectioning, the tubule with which the inactivated glomerulus is connected. Mechanical distention with fluid of the lumen of the tubule flattens the epithelium comparably with that observable in a diuretic kidney. The functional significance of this alone is doubtful. Certain experiments on the frog involving ureteral injections indicate possibilities for a pyelovenous backflow.