1. The wall of the proximal convolution in chronic canine nephritis is composed of various types of epithelial cells which can be recognized as definite structural types from their cytological characteristics.
2. The function of these cell types, as tested by their reaction to the administration of trypan blue, varies with their structural constitution.
3. As a result of the varied cellular content of its wall the abnormal proximal convolution handles trypan blue by mechanisms which differ both quantitatively and qualitatively from those of the normal convolution.
4. A distinguishing characteristic of the decompensated kidney in chronic canine nephritis is the inability of its epithelium to concentrate trypan blue within its cells and to prevent diffusion of the dye from the lumen into the tubule wall.
5. It follows: (a) (from conclusion 3), that it cannot be assumed that the renal mechanisms concerned with other substances are not unaltered and that comparisons of blood and urine concentrations (clearances) have similar significance in the normal and nephritic kidney; (b) (from conclusion 4), that tubular dysfunction may play a part in the ultimate failure of the compensating kidney in all forms of chronic Bright's disease where the tubule walls are similarly affected.