This paper provides a proposal for the analysis of the rhetorical structures of twenty-five texts in Spanish. These texts, chosen at random, are representative of written discourse in the field of biomedicine, and belong to the subgenre of opinion: the Carta al director. The aim is, consequently, to check if the same schema is found in each text. Within this study the rhetorical structures of the selected texts are analysed by applying a modified analysis of scientific texts set forth by the linguist Paltridge (1997), based on keys and influenced by the Genre Analysis Theory (Swales, 1990). In the method proposed at least one key is used to represent each sentence of the selected texts so that structural content in the corpus can be clearly identified.