Publisher Summary: This chapter discusses that the intact mammalian kidney double perfusion is possible only by applying micropuncture techniques. It is obvious that by double perfusion, a three compartment (lumen-cell-interstitium) approach is applicable to the tubules in situ: Either in steady state or after rapid solution changes the concentrations of most substances in the compartments (i.e., the compartment volume) can be measured. The same holds for electrical parameters, potential difference, DC and AC resistances. To study contralumenal transport—that is, from the interstitium into conical, mainly proximal tubular cells—a method was devised that causes the lumena to collapse so that only two compartments, extracellular and intracellular space, are present. For this purpose, the renal artery and vein are isolated from the ureter so that they can be clamped underneath the plastic cup with a U-shaped hook that bends the blood vessels against the edge of the cup when a weight is attached to the hook.