Androgen production by both testes and adrenals decrease in old age; this is partly the consequence of a decrease in the metabolic clearance rate but plasma levels as well as their response to human chorionic gonadotropin (HCG) and adrenocorticotropic hormone stimulation, respectively, do also decrease. As far as testicular androgen levels are concerned, there exists a large interindividual variation of plasma levels even in old age, some elderly persons having levels comparable to those found in young adults. Others have clearly decreased levels. Causes contributing to their variability are general health, physical and sexual activity, smoking habits, obesity, genetic factors, and intake of drugs. Although in exceptionally healthy persons, both physically and sexually active, testosterone levels may, therefore, not decrease in old age, in the elderly population at large, such a decrease does occur, even when all other factors influencing their levels are controlled. The decrease in testicular androgen secretion appears to have a primary testicular origin as luteinizing hormone levels are slightly, but significantly, increased and the response to HCG decreased.