Across the twentieth century, hormones came to be seen as messengers of sex. As such, they figure today as key players in maintaining the biological life of human and non-human animals. Sex hormones are understood as essential both to the development and maintenance of healthy foetal, child and adult bodies and to the very possibility of sexual reproduction. This chapter examines contemporary technoscientific representations of the messaging actions of sex hormones. Working with contemporary theories of scientific knowledge production, it suggests that explanations of hormonal messaging contain unrealised potentialities for recognising the significant interweaving of ‘the social’ and ‘the biological’ constituting this messaging. In focusing attention on the intricate and microscopic patterns of hormonal flows, biologists and physiologists attempt to leave the social aside – to create, as Haraway puts it, ‘respectable distance’ between these explanatory categories. Close examination, however, shows that this ‘clean … epistemological space’ is never fully achieved. Indeed, ‘nothing so productive could be so simple’: biological explanations of hormonal actions are always articulated with the social, both in their epistemologies and in their production as technoscientific knowledge.