Unilateral pyelonephritis was induced in the rat and micropunture techniques were used to assess nephron function in individual nephrons from chronically diseased and normal kidneys in the same animals. Conventional clearances were performed simultaneously in the same kidneys. The clearance data indicated that the chronically diseased kidney of the rat, like that of the dog and man in a nonuremic environment, behaved in a predictable and organized manner. Analysis of data from micropuncture of the proximal tubules of individual nephrons from the chronically diseased kidneys indicated that salt and water reabsorption proceeded in a manner indistinguishable from that in nephrons of the contralateral control kidneys in the same animals. There was no evidence of intrinsic lesions affecting salt and water transport in the portions of the nephrons examined.