This article analyses the growing importance transnational, non-statal actors has taken in international relationships. Specifically, the author explores the case of environmental actors discussing a casestudy: Greenpeace’s action in Antarctic, during the 1980’s. The main hypothesis states that transnational, non-statal actors, just like environmental groups, forces a reviewing of the realistic approach to the theory of international relationships, according to whom the monopoly of those international relationships belongs to the national State. According to that hypothesis, the emergence of transnational, non-statal actors defies the realistic notion, and opens space for an international system that is plural for its actors and agenda. The conclusion sets for a growing commitment, at national and international levels, for that kind of non-statal actor, in an arena different from the stractegic-military one. So, this study hasbeen made considering the interaction of Greenpeace and the State system organized to the Antarctic Treaty.