Abstract  This paper undertakes a critical examination of the rationalisation of British dairy farming in the early twentieth century, with a particular focus upon the emergence of milk yield recording as a vehicle of rationalisation. The historical analysis is used to rework and rethink the concept of rationalisation itself, by conceiving it as a disciplinary technology of ontological purification, which reconfigures the relations between humans and nonhumans, and between humans and animals in particular. In this way I seek to integrate contrasting approaches to modernity, showing how the core sociological narrative of rationalisation can be re-worked in terms of a Foucauldian conception of disciplinary power and a symmetrical or ‘actor-network’ approach to ontological politics.