In common descriptions of the world trading system, MFN and national treatment embody the principle of non-discrimination. Reciprocity is mostly regarded as a remnant of mercantilist trade policy. This paper argues that reciprocity is the more fundamental constitutional rule, because it reflects the unanimity principle enabling countries to identify Pareto-improving changes of trade policy. I reconstruct the implicit constitution of world trade by applying some methodology of positive constitutional economics. A major result is that in the Uruguay round a fundamental constitutional change took place from general reciprocity to “status quo reciprocity”, which entails the possibility of Pareto-deteriorating institutional changes. Copyright Springer Science+Business Media, LLC 2006Positive constitutional economics, WTO/GATT, Reciprocity, Regionalism, MFN, F02, F13,