The combined biochemical and ultrastructural study of the changes in the renal medulla of the rat depleted of potassium emphasizes the clarification of the pathogenesis of this lesion. Tracer materials will be followed as they are incorporated into microsomal and lysosomal components using high resolution electron microscopy and tissue fractionation with lipid analysis. Similarities of the rat lesion to the recently described medullary human lesion will be sought.