SYNOPSIS. The ability of different sex hormones to activate social signals can provide important clues to the biochemical mechanisms underlying these signals. A pattern of hormone specificity in which testosterone (T) and estradiol (E), but not dihydrotestosterone (DHT), are effective suggests that conversion (aromatization) of T to E in the brain may be involved or required; a pattern in which T and DHT, but not E, are effective suggests that conversion of T to DHT may be involved. The hormone specificity of social signals in diverse species of vertebrates is reviewed. Aromatization seems to be of widespread behavioral significance in mammals andbirds. A role for conversion of T to DHT is suggested for some signals. Aromatization of T mayalso be important for the activation of adult female behavior in mammals and lizards, and for the early organization of behavior in mammals and birds. Patterns of hormone specificity differ both across species for a given social signal, and within a given species when different signals are compared. An attempt is made to integrate these findings by relating patterns of hormone specificity to hormone levels, steroid receptor and enzyme concentrations and distributions, signal function and dimorphism, and phylogenetic status.