In this work I deal with the conceptual motivation of the grammaticalization of the items mas, porém, todavia, entretanto e no entanto, which are classified as adversative conjunctions by the majority of the Portuguese Grammars. Though such classification is very discussable - for all of these items but mas present nontypically- conjunctional syntactical behavior - we consider the fact that all of the items cited above have become more grammatical in some manner since their medieval origins until the present days. Hence, the grammaticalization process is understood as that one in which the lexical items and constructions come in certain linguistic contexts to serve grammatical functions or how grammatical items develop new grammatical functions (HOPPER & TRAUGOTT, 2003). This work focuses the quest for the conceptual elements which could have motivated this process. Works such as Barreto´s (1999) state that the motivation for the grammaticalization of the items being studied in this text was a metonymic one, carried out by the influence of semantically negative elements which appeared next to them in the Medieval Portuguese. The objective of my work is to investigate why these items were massively found, as it seems, in contexts with negative-sense particles. From the reading of works such as Vogt´s & Ducrot´s (1980) and Sweetser´s (1991), I understand that mas was found in such contexts due to a metaphoric motivation and that the adversative relation for whose establishment it contributed occurred mainly within the epistemic domain of language. This very analysis is extended to the other items, which, according to what can be verified through the analysis of samples of the Medieval Portuguese, had the tendency of being used in contexts in which two groups of linguistic information, put into an adversative relation, were delimited, contributing for the text cohesion. This adversative relation, if not grammatically marked, was, even though, inferrible through the analysis of the textual relations which took place in the linguistic meaning plain, especially at the epistemic level. Thus, negative elements were possible in these contexts and with them, also, the elements responsible for the textual cohesion.