This paper discusses the conditions of birth and growth of business networks in the veterinary drugs sector based on Brazilian examples examined by the authors. The beginnings of business networks are approached from two paradigms, one advocating the idea that the birth and growth of networks depend on some variables’ historical processes which define social networks, such as trust, cooperation and commitment, and another postulating that these attributes are not necessary for the formation of networks, it being sufficient to have only consistent economic expectations and a proper agreement. The aim of this paper was to assess which of the two network paradigms (the social paradigm and the business paradigm) best explains the birth of two networks in the veterinary drugs sector. An exploratory study was conducted, using the case study methodology, to collect and analyze information from these two types of network. The analyses of the two starting networks (one a failure and the other a case of initial success) confirm the importance of trust, cooperation and commitment in relationships. The final discussion indicates some theoretical and managerial paths that can be followed in social networks as a backdrop for business networks in the agricultural sector.