Following castration, the nucleus of the dark cells of the subcortical portions of the collecting tubules in the rat kidney showed two types of morphological alterations: 1) Decomposition of the nuclear membrane in a circumscribed area. This phenomenon is independent of the sex of the animal and becomes more frequent up to the 30th day after castration. Occasionally, decomposed nuclei can also be observed in controls. 2) Giant nuclear bodies. These inclusions occurred only in castrated males. Early stages were already observed at the 10th day after orchiectomy, whereas fully developed giant nuclear bodies could not be demonstrated until about the 20th day after castration.