The hypothesis worked out in the article is that realism and postpositivism in international relations theory have been pervaded by notions of rationality inadequate to contemporary philosophy, and that this has largely lead both paradigms into comparable problems of ethical relativism. The article offers a study of the flaws of instrumentalism in realism and of the debasement of the role of rationality in postpositivism that these ethical problems result from. The contributions that contemporary conceptions of rationality could offer to the discussion of ethical problems in international relations are also pointed out in the article. Refs 27.