Despite the abundance of career education interventions and information on career opportunities, the search for information process is not necessarily performed very thoroughly, and thus persons, choosing a profession, do it in the simplest way, being unable to properly explain their decisions afterwards. In choosing a career, it is crucial to think through potential alternatives and support them by arguments. That is why it is important to discuss the process of reasoning, which evolves coherently and can be characterised by certain elements. The following research question was raised in this study: “How the reasoning of pupils’ on career choice is distinguished at a comprehensive school?” In this case the main accent of the research is focused on the situation of career choice reasoning at Lithuanian gymnasiums. The research aim was to disclose the phenomenon of the reasoning on pupils’ career choice at a comprehensive school. For this article, the data have been collected by methods of literature review and individual semi-structured interviewing. Qualitative content analysis has been performed while processing the information acquired from semi-structured interviews. Findings from literature review showed that the reasoning is assessed as conscious, aim-oriented process of conclusion drawing, by means of which it is pursued to evaluate alternatives, solve problems and make decisions by referring to certain information, facts and features, personal characteristics and beliefs. Reasoning based on stimuli emergence is the aim-oriented process, functions of which are based on information, knowledge and experience. This needs mental abilities, language. Reasoning is related to personal inclination and habits. Empirical findings revealed that the phenomenon of career choice at school is not unanimously understood and does not have clear structure. The character of pupils’ reasoning is closely related to emergence of certain stimuli. The external stimuli that induce the reasoning on one’s career choice, which were mostly emphasized by the informants, allow considering that here the school educational environment plays the important role.