1. A method is described for the production of a nephrosis in one kidney of an experimental animal. The normal kidney in the same animal is available for a control. 2. The nephrosis produced by the injection of 7.5 per cent sodium tartrate is limited to the proximal convoluted tubule. There is no histological evidence of glomerular damage. 3. This damage results in a disturbance of water and chloride excretion as well as a decrease in the clearances of urea, xylose, inulin, creatinine and phenol red, and is observed under conditions of no diuresis, sugar diuresis and salt diuresis. 4. Excreted ferrocyanide is found in the cells of the proximal tubule of the damaged kidney. It is not present in these cells in the normal kidney. This is taken to be evidence of back diffusion of ferrocyanide through the damaged cells. 5. The decrease in clearance of creatinine and inulin, which in the dog represent glomerular filtrate, may therefore be explained by back diffusion and is not necessarily due to glomerular change. 6. The phenol red clearance from the damaged kidney is diminished in relation to inulin. This is further evidence of depressed tubular activity.