This research aims to show the dynamism of the outil linguistique (Auroux, 1998) of grammar by studying the theoretical change present in it, focusing solely on the first Portuguese grammars. We examine the theoretical scenario in which Portuguese grammar was constituted and developed, from the 16th to the 18th centuries. We analyse of the sixteenth century only two grammatical works were produced (Oliveira 1536 and Barros, 1540); and of the seventeenth century only a grammatical method for language teaching (Roboredo, 1619); while in the 18th century, production increased, although, so far as is known, fewer than a dozen works were written, among which we selected two for analysis (Argote, 1725; Fonseca, 1799) for their relevance in relation to the object of our study, the theoretical change. The theoretical movement seen in the grammar has a direct impact upon the grammatical description, the grammar classes and their accidents, as we intend to highlight. Mots clefs Grammaires protugaise, changement théorique, description du portugais.