This paper aims at linking the AI notion of multi agent system with the notion of LEGAL RELATION [Allen & Saxon 95]. The paper is based on the idea that legal rules concern actions to accomplish or to exclude and states to achieve or to avoid or actions for changing existing legal relations. The body of the rule establishes who and under which conditions must respect the obligation. The notion of ‘obligation’ has been defined elsewhere [Boella & Lesmo 01] and will be reviewed here. In this paper it is used as an ontological basis to define the LEGAL RELATIONS appearing in the A-Hohfeld language, and the concepts of ‘bearer of the obligation’ (who undergoes the rule) and of ‘normative agent’ (who watches on the rule) are connected to an ontology of legal entities.