Alterations of cortico-papillary electrolyte gradient of the kidney can now be evaluated by continuous recording of tissue electrical admittance (a reciprocal of impedance), a variable changing directly with tissue concentration of free ions. Measurements are performed with needle electrodes placed in different zones of the in situ kidney, and it is then possible to follow the dynamics of changes in medullary electrolyte concentration. This approach, described recently from our laboratory, has so far been used in large animal species (dogs and rabbits). The present work describes an adaptation of the method for its use in the rat, accomplished by reducing the number and size of electrodes and a modification of their geometry. This satisfactorily minimized kidney damage secondary to insertion of electrodes. The information derived from admittance studies in the rat can be readily compared with numerous data on tubular transport of electrolytes obtained in this animal species.