Kantian ethics is based on a metaphysical conception of autonomy that may seem difficult to reconcile with the empirically-based science of psychology. I argue that, although not formally developed, a Self-Determination Theory (SDT) perspective of ethics can broaden the field of Kantian-based moral psychology and specify what it means, motivationally, to have autonomy in the application of a moral norm. More specifically, I argue that this is possible when a moral norm is fully endorsed by the self through a process of internalization that is energized by intrinsic motivation and is facilitated by the fulfillment of the basic needs for autonomy, competence and relatedness. The conditions of the fulfillment of these needs may even implicitly reveal which norms will be integrated and treated as moral norms. I conclude that SDT offers a motivational approach that is useful in understanding the development of moral norms.