Participatory research methods are increasingly used in the agricultural research for several years all over the world. More precisely, participative methodologies can be performed to design and assess sustainable farming systems. Scientists are used to build and manage research on-station experiments with variable consideration to the professional agricultural world immediate stakes. Thus, they collaborated through different ways from information exchanges to service agreement. Some of these experimental stations have now decided to formally involve agricultural actors in the design, assessment and management of their tested farming systems. However, this implies to clarify the objectives of these participatory methodologies such as their conditions of implementation. What is the interest of using participatory methods for designing farming systems for the agronomists? How to precisely describe the “participative effort” and the adopted methodology? What kind of knowledge is produced by participatory methods? How to reduce the inequalities shaped by a participatory mechanism initiated by researchers? To address these questions, we developed a reflexive and interdisciplinary analysis involving social scientists and agronomists, about a research program that uses participatory methods. Through this program, agronomists aim at designing and experimenting protected vegetable farming systems to reduce pesticides uses. Therefore they develop participatory methods, which can be declined in three main points: Producers or experts (scientists, extension agents) were punctually contacted at different stages of the project. These exchanges contributed to clarify the set of goals and constraints that producers have on farm. In addition they contributed to collect knowledge and know-how to understand biological processes or to manage the experimented crops. Design workshops were implemented one day every semester. Researchers, technicians, farmers and extension agents gather to explore in collective hitherto unseen ways to design innovative practices and management strategies. “Tracking” of on-farm innovations developed by producers was implemented. It consists in looking out for innovations designed by farmers and analyzing them to get empirical references useful to guide the design process (on farmer’s practices, motivations to develop them…). While performing this participatory program and during the design process, innovative combinations of practices are experimented on station. In the same time, social scientists realized interviews with technicians and researchers (15) of the experimental station where takes place this participatory program. The interviews aim at analyzing the context of development of the participative approach and focus on the transformation of the experimental activities, the research themes and the nature of partnerships within the experimental station. A social scientist also observed the participative design process and frequently asked agronomists to explicit their choices to maintain a reflexive dynamic. As social scientists carried out these interviews, observations and regular interactions, they were both observers and actors of the participative process. They contributed to formalize the participative method and its evolution through “live feedback” or retrospective analysis and to take in account social dimensions. With this methodology, we tried to open-up the black box of a specific participatory design process. We aim at explaining why and how agronomists working on a system experiment in an experimental station chose to formally involve technicians, farmers, specialized scientists and extension agents in their design process. We made explicit how the interactions between social scientists and agronomists fed the evolution of the participative research program and helped to better describe the ongoing participatory process. 2 The two main questions leading agronomists to get involved in a participative approach are: How to design and assess sustainable systems responding to the challenge of reducing pesticide use at the field scale while considering food systems constraints? How to design sustainable systems with scientific knowledge gaps about vegetable production systems? Therefore agronomists developed a step-by-step design methodology, based on the accumulation of learning to improve the practices experimented. This method allows identifying and integrating, step by step, new research questions, knowledge and know-how (coming from both experiments and partners)since it maintains open the realm of the possible in terms of design choices. To guide this design process, they also organized recurrent workshops to gather a fixed collective of stakeholders having experiences complementary to the project’s scientific carriers. Working with this collective allowed the agronomists to go over “concepts”, to face it to “farmer’s realities” (normative recommendations, logistic or economic constraints on farm, technical uncertainty…). It invited agronomists to consider multiple dimensions of farming systems and enabled designing and experimenting combination of innovative and operational practices. Prototypes and results of the experiment were discussed within the group during these workshops. This allowed bringing scientific results closer to the “practice” and identifying new scientific knowledge gaps. These gaps were also identified through the experiment and dealt with specific practices or combination of practices (decisional or biological processes). To fill them, agronomists collaborate with specialized scientists, extension agents but also with farmers who are thought to develop relevant innovations on their farms. However, while implementing this participative design process, agronomists faced two challenges. The first was to clarify the kind of knowledge produced by participatory design methods. The social science analysis led to consider that the extension agents and farmers provide “experiential knowledge” about the coordination of different “socio-technical growth factors” within specific localities which is mixed with the scientific knowledge of the agronomists. Agronomists then deal with on-station experimental advantages (e.g. taking risks is more possible than in on-farm experiments) and constraints (e.g. dealing with the overall activities to coordinate in the station) as they stay the only decision-makers and monitors of the experimentation. Therefore, even if the designed systems are discussed within the group of participants, we cannot really speak about a “co-design” process. However, we can consider this initiative as a step towards “experiential science”. The second challenge was to build and maintain a participative program where scientific and nonscientific interests meet. Working on sociological hypothesis and analysis (e.g. about the interactions between the various participants and about the science-society relationship), we tried to establish a balanced relation between agents from the experimental station, farmers and extension agents where everyone can express his expectations. Thus, the agronomists paid attention to present their own practices of research and to encourage extension agents and farmers to share their experiences and innovating farming practices. Thus, the design process permanently evolve following a non-linear “design-experiment-assessment process”. In this case, the system experiment appeared to be an interesting “intermediary object” to build and maintain interactions with partners since it provides results and offers a concrete way to valorize the knowledge exchanged. In conclusion, this reflexive analysis of a participative design process explained how the agronomists were able to bring their work closer to farmer’s realities working with partners. Moreover, it showed this process contributed to design and manage the innovative systems experimented helping to accumulate scientific and operational knowledge. It also revealed how interactions between agronomists and social scientists generated a fine-grained description of an ongoing participatory process. This interdisciplinary analysis finally highlights the characteristics of a non-linear and dynamic participative design method. This opens the door to an “experiential science”, which could be the future of the knowledge production on research experimental station.