AT the cellular level different mechanisms have been demonstrated or postulated for the transport and processing of proteins in the epithelium of the proximal tubule. Eight pathways are summarized and numbered in Figure 13: 1. The main transport pathway for proteins consist of endocytosis from the tubule lumen and subsequently lysosomal digestion. 2. Proteins do not pass through the junctional complexes into the intercellular spaces under physiological conditions but may do so in very early embryological stages or if the luminal hydrostatic pressure is experimentally increased. 3. Certain small proteins or polypeptides may be catabolized by membrane digestion at the surface of the brush border membranes. 4. Proteins may normally diffuse retrograde into the intercellular space through the basement membrane and then return unaltered to the peritubular space. 5. Endocytosis from the peritubular cell surface is rare and quantitatively insignificant. 6.-8. There is no convincing evidence for the existence of transcellular transport of intact protein, either directly through the cytoplasm, or by peritubular release of protein-containing lysosomes or endocytic vacuoles.