Age-related changes in the testis were studied histologically in dd-mice from 2 months to 2 years of age. After 6 months of age, vacuoles appeared first singly and later became clustered in the seminiferous epithelium. With the appearance of the vacuoles, the epithelium started to release spermatids and spermatocytes into the lumen. Multinucleated giant cells occasionally appeared in the epithelium and were also released to the lumen. The epithelium, when markedly depleted of germ cells, was composed mainly of Sertoli cells. The lumen of the atrophied tubules occasionally contained accumulations of macrophages, sperm aggregations, and nodules of a homogeneous material covered with flattened epithelial cells. The basement membrane surrounding the atrophied tubules thickened corresponding to the degree of atrophy. The atrophied tubules initially appeared in patches and then spread throughout the testis. Leydig cells increased in amounts with age. The increase of Leydig cells was distinct around severely atrophied tubules with a thickened basement membrane. These changes are discussed in comparison with autoimmunized testes which show similar histologic changes.