Based on the concept of conjunction as presented by Halliday and Hasan, this research, which has a psycholinguistic character, aims to verify the processing of internal and external causal conjunctive relations, as well as the role of connectives in this processing. For this, it was first made an exploratory activity of writing with students from 7th and 8th grades of elementary school , to see whether in this level of schooling they already use internal and external causal conjunctive relations, and if they do it lexical- grammatically through connectives . Having confirmed the use of these relations, we performed the off -line reading comprehension test and cloze test in classes from the 8th and 9th grades of Elementary Education, respectively. The result of statistical analysis of the data indicated that the causal conjunctive relationships under investigation have different processing costs. In the comprehension tests, where the task of the participant was the verification of statements about the content inferred from the text, external causal conjunctive relations were more easily processed . In the cloze test , in where the establishment of relations between sentences is done by the student and, thus , depend more on its extra-linguistic knowledge and on the type of connective that can appropriately express such semantic links, external relations seem to have brought more cost to the processing than the internal ones. In the case of the internal ones, beyond identifying the argumentative character of the text, linguistic cues, such as the use of modal resources, especially adjectives and the use of certain verb tenses and moods, may have favored the recognition of the type of relation (regardless of having had a more effective understanding of the text). Regarding the presence or absence of connective it was not observed a main effect of this variable in any of the experiments, which can, at first, be related to the strength of the causal relationship between clauses, which even in the absence of connective can be retrieved by the student in the texts used. This work brings a contribution to the question of reading in the classroom, particularly with regard to text comprehension of argumentative nature, in which internal and external causal conjunctive relations appear substantially, and should therefore be best addressed in class from all subjects, exercising an interpretation that goes beyond the surface of the text.