OF THE various hypotheses advanced to account for male pattern balding, most have been rendered untenable by the simple demonstration that testicular hormones are a prerequisite for the common patterned alopecia of the male. As shown by Hamilton, castrate or eunuchoid males are free from pattern balding, while testosterone can induce hair fall in genetically susceptible castrates or eunuchoids. Moreover, hair fall can be checked by castration, so that postpubertal castrates exhibit less hairline recession than normal controls of comparable age.*Therefore, tight headgear, exposure to the elements, and lack of hair care no longer need be blamed for the patterned loss of hair in the male, and ketonic steroids rather than obscure micro-organisms seem to be the mediating factor. What is not clear is just how masculinizing sex hormones act upon the follicles or the supporting tissues to bring about patterned loss of hair. In an attempt to