Changes occurring in lumbar and lower thoracic vertebrae of aging dwarf mice (dwdw) were compared with those found in non-dwarfs of the same strain (Dwdw or DwDw) and of strain C57B1. Dwarf vertebrae, besides remaining smaller, develop less completely and more slowly than those of non-dwarfs. Retarded development is indicated by the presence of non-resorbed epiphyseal cartilage, by diminished removal of cancellous bone from the vertebral bodies and possibly also by the failure of osseous replacement of the cartilaginous end-plates. The slowing of the aging process manifests itself in decreased degeneration of the growth zones, delayed loss of mucopolysaccharide from the matrix of the articular cartilage and by an attenuated tendency to develop spondylosis.