The thesis of “contextual rationality” produced during decade of 1980 constitutes a truly field of research opportunity not fully explored inside the Institutionalist theoretical ground. Aligned with the Rational Choice Theory which postulates that actors aim to maximize their own gains, the contextualization of this behavior, put into practice by the Brazilian political parties, is illustrated by the strategy of making aliances with different parties. However, the difference in campaign resources and goals between the bigger and smaller parties equally constitute the adoption of different strategies in the electoral game. The present thesis aims to analyzing the behaviour of the Brazilian political parties regarding electoral coalition formation for the State and National Legislatives proportional representation elections. Postulating that bigger parties have less incentives to form electoral coalitions with parties of equal size (based on last elections electoral outcomes) since that would go against our hipothesis of rationality. At the same time, rationality is not to be understood strictly bearing in mind other conditionalities of the Brazilian electoral system such as the degree of competitiveness of different electoral districts and the formation of electoral agreements for the majoritarian disputes for the State Executive. As we noticed, in fact strong parties do form alliances with other parties equally strong specially in the case of the elections for the Chamber of Deputies. We have also observed that both different district magnitudes as well as the establishment of electoral alliances for the State majoritarian elections constitute relevant variables in the strategy for party alliance formation.