The paper studies the observations on the Portuguese language in a corpus of grammars of Castilian and apologetic treatises on this language published in the 16th and 17th centuries, for which analysis parameters have been determined. The results, as far as the grammaticography of Castilian is concerned, reveal the little importance given to Portuguese as a reference language for the description of Spanish, as well as a consideration of this language as subordinate to Castilian. As far as the apologetic treatises are concerned, there are also few references to Portuguese. Where Portuguese is mentioned, two approaches can be identified: i) the identification of Portuguese as a dialect of Castilian; ii) the defence of these as different languages.