The first section of this paper examines the level of environmental regulation on preferential trade agreements (PTAs) signed by Mercosur members (Argentina, Brazil, Paraguay, and Uruguay). Using quantitative methods, this paper finds that PTAs signed by Mercosur members have fewer environmental provisions than PTAs from other countries or blocs, regardless of whether these other countries are from the global North or South. To understand why that is the case, the second section of this paper turns to the domestic politics of a Mercosur member, Brazil. Previous literature has pointed out that Brazilian agrarian elites are a powerful lobby in Brazil, concentrating in the Ruralist Bench in Congress. These elites oppose environmental regulations since their production is environmentally damaging. Given that these elites produce mainly for exports, they would be especially interested in lobbying for trade agreements with low environmental standards. Using qualitative methods, this paper found mobilization in the Brazilian Congress to quickly approve PTAs without discussions regarding their provisions. Furthermore, it found a lack of mobilization from the Environmental Bench and civil society against the low level of environmental regulation of Brazil’s PTAs. These findings point out that those interested in mitigating climate change and advancing sustainable development are missing a critical arena for action, the Brazilian Congress. Lowering the effects of international trade on the environment is critical to reaching climate change and sustainable goals. This paper has shown that Mercosur’s PTAs have an exceptionally low number of environmental provisions, which must be addressed. Furthermore, it has identified a key arena of advocacy and lobbying that is being left empty by civil society, the ratification of Mercosur’s PTA in the Brazilian Congress. By identifying such an arena, this paper has contributed to future research, policymakers, and civil society to lower the effects of trade on the environment and advance sustainable development.