Since its founding in 1927, the Permanent Assembly of Chambers of Agriculture (Assemblée permanente des chambres d’agriculture, APCA) changed several times its role and social profile. Its history begins while the farmers unions are almost celebrating their fiftieth anniversary. The APCA then starts a long quest to set itself as an intermediary between the agricultural professional organizations and State structures. Although this institution is exclusively a consultative one in the 1930’s, it progressively tends to become a protagonist of the agricultural development policy in the 1960’s-1970’s, offering technical and administrative support to the local chambers of agriculture, and developing a public advisory capacity as well as its own publishing policy. Prosopographical and micro-historical methods allow us to demonstrate that this process of institutionalization has been the long-term task of some farmers leaders and their managers, men born at the beginning of the twentieth century, and that its success is the result of slow adapations of all the players involved. A relational database system allows us to follow the social trajectories and to localize in the nebula of professional and political networks some 8.000 members of the chambers of agriculture of this period, as well as their 500 presidents. From the Third Republic to the Peasant corporation of the Vichy régime, and then from the Liberation in 1944-1945 to the agricultural modernization of the “Trente glorieuses”, this history is also a way to enlight how agrarism as a pattern of control could endure while productivism as a new paradigm was asserting itself.