In normal men, advancing age is accompanied by increase in the incidence and extent of baldness. In eunuchs who were castrated prepubertally and given androgenic treatment at a later date, those who are in the second decade of life when treatment is begun tend to lose hair slowly over a period of years, as in most normal young men of their age. In contrast, the eunuch who reaches the sixth decade of life before receiving androgenic treatment loses his hair within a few months ofthe beginning of treatment. Evidently the susceptibility to alopecia increases with age but is not expressed in the absence of inciting agents like androgens. These genetic, endocrine, and aging factors are interdependent. No matter how strong the inherited predisposition to baldness, alopecia will not result if inciting agents, such as androgens, are missing. This is illustrated by the luxuriant scalp hair in old eunuchs who were castrated prior to sexual maturation. Neither are the androgenic inciting agents able to induce baldness in individuals not genetically disposed to baldness. The augmenting action of aging has been referred to above