Membranous and vacuolar inclusions have been identified within the nucleus of mammotrophs in the pituitary of female Mongolian gerbils but are absent in adult male and newborn gerbils of both sexes. Membranous inclusions are observed in focal areas of the nucleus and are frequently in proximity to the inner nuclear membrane. Vacuoles are surrounded by a single membrane and the matrix is structureless and either flocculent or clear. An intermediate form of nuclear inclusion is membrane-bound and contains typical cytoplasmic organelles. It is suggested that degenerative changes in inclusions originating in the cytoplasm may produce large structureless vacuoles. The increased number of inclusions in testosterone-treated animals may be a response of mammotrophs to alteration in synthesis or release of hormone.