A definitive feature of Kant's moral philosophy is its rationalism. Kant insists that moral theory, at least at its foundation, cannot take account of empirical facts about human beings and their circumstances in the world. This is the core of Kant's commitment to ‘metaphysics of morals’, and it is what he sees as his greatest contribution to moral philosophy. The paper clarifies what it means to be committed to metaphysics of morals, why Kant is committed to it, and where he thinks empirical considerations may enter moral theory. The paper examines recent work of contemporary Kantians (Barbara Herman, Allen Wood, and Christine Korsgaard) who argue that there is a central role for empirical considerations in Kant's moral theory. Either these theorists interpret Kant himself as permitting empirical considerations to enter, or they propose to extend Kant's theory so as to allow them to enter. With some qualifications, I argue that these interpretive trends are not supported by the texts, and that the proposed extensions are not plausibly Kantian. Kant's insistence on the exclusion of empirical considerations from the foundations of moral theory is not an incidental feature of his thought which might be modified while the rest remains unchanged. Rather, it is the very centre of his endeavours in moral philosophy. If we disagree with it, I argue, we have grounds for moving to a distinctly different theoretical framework.