Male mice of strain C57BL Jax6 were treated with small doses of testosterone during growth and observed to the age of eighteen months. In mice thus treated, aging changes in the joints were accelerated and the incidence of osteo-arthrosis was significantly increased, as compared with conditions established in untreated mice. The results reported thus provide additional evidence for the importance of aging changes in the development of deforming arthrosis. The aging and arthrosis-promoting effects of testosterone are thought to be related to its growth-promoting effect and therefore to its protein-anabolic action. The male sex hormone thus seems to be responsible for the higher susceptibility of the male mouse to degenerative joint disease as compared with that of the female.