A study was conducted on how scientific laboratory lessons contribute to building arguments, both cognitively and socially. The population consisted of 12 second-year pre-service teachers at a college of education specializing in the teaching of biology and chemistry in middle school. The study examines the nature of the arguments students raise when conducting a laboratory experiment and how conducting an experiment contributes to curricular science teaching in the laboratory. For this purpose, a number of methodologies were used: observations, experiment reports, and statistical analysis. The findings showed that groups conducting an open-ended experiment made more claims in their discourse than did groups that conducted a confirmatory experiment and that the level of argumentation in the open-ended experiment group was higher than in the confirmatory experiment group. Interestingly, despite the significant difference in the level and quantity of arguments in the discourses in the two types of experiments, no significant difference was found regarding the level of argumentation in the two groups' experiment reports. However, students confirmed the great importance of scientific laboratory lessons to the development of argumentative thinking, meaningful learning, and the students' relationship with their classmates.