This contribution analyses the notion and practical application of the concepts of representative democracy and constitutionalism in relation to national parliaments within a European Union that is understood not in the classic constellation of hierarchy but rather in that of heterarchy. A specific focus of investigation is the post-Lisbon function of national parliaments, as attested by the Fiscal Treaty and as conceptualised in a variety of models of EU constitutionalism. However, there is as yet no sophisticated methodological approach for assessing the direct and indirect interactions between national parliaments and EU institutions. There is very few evidence of national parliamentarians acting as European actors. In order to test the theoretical premises, the article delves into the political praxis and examines the application by MPs and senators of scrutiny instruments regarding a number of EU dossiers that were the object of cross-level discussions within the framework of COSAC and the Barroso Initiative. The objective of the inquiry is to test the hypothesis that national parliamentary scrutiny of EU matters does not occur as an isolated domestic process but as a process that is interdependent with those unraveling at the EU level and that such scrutiny may have implications for the European Union’s democratic legitimacy.