The purpose of this article is to reconstruct, at a micro-level, the practices of food trade in a large French city during the seventeenth century and to explore the way in which economic actors deal with information and uncertainty. The first part of the paper shows how, in a context of population growth and frequent subsistance crises, urban authorities strive to extend their control over food supply, prices and the quality of food products, in order to reduce uncertainty and to protect consumers. However, this institutionalization process is slow and imperfect: the second part of this article deals with the way in which some institutions and rules, which are precisely created to clarify the conditions of exchange, are occasionnally diverted from this first aim by economic actors and therefore contribute paradoxically to blur market information. The third section of the paper finally explores the various kinds of economic rationality and shows how, in contexts of rare and imperfect economic information, some actors precisely take benefits from this uncertainty by using social links and networks to their best advantage.