Market definition according to reducing pollution at least cost whith transferable rights for manure spreading - Concepts from transferable discharge permits were applied to the context of nitrogen surplus of animal origin in Brittanny. We simulated markets for spreading rights in some local surplus zones. The demand function was made by aggregating the costs of manure processing. We calculated rights prices, land application and industrial processing of manure, and the costs savings due to the market in comparison with current regulation. Perspectives of application to nitrogen policy are discussed.