Interpretações administrativas aderem à lei?

The article analyses how administrative law theory has been, throughout the times, treating the discussion about whether custom (or administrative practices) is a source of law. Moreover, it defends the thesis that a longstanding statutory interpretation, consolidated by administrative practices, must be considered as a part of the interpreted statute, for the purpose of analyzing the validity of acts, contracts and processes produced while the longstanding interpretation was in force. Finally, the article argues that, although in theory longstanding interpretations can be revised for future cases, the interpreter who intends to revise them has a supplemental ar- gumentative burden: the one of empirically analyzing the consequences of the longstanding interpretation, proving in a strong way that, in the real world, those consequences are, or have been, negative. In this manner, the motivation that can lead to a new, opposing interpretation to the longstan- ding one, cannot be of an exclusively abstract character.