Motivation crowding studies have demonstrated that external interventions can harm effort and performance through crowding out of intrinsic motivation, when interventions are perceived as lack of trust. However, motivation crowding theory also presents a much less investigated crowding in effect, which occurs when external interventions increase intrinsic motivation. This study empirically tests the motivational effect of a specific external intervention and its associations with the perception of the intervention. We draw on a cluster randomised stepwise introduction of a mandatory accreditation system in general practice in Denmark combined with baseline and follow-up questionnaires of 1146 GPs. Based on a series of mixed effects multilevel models, we find no evidence of motivation crowding out among surveyed GPs, although most GPs perceived accreditation as a tool for external control prior to its implementation. Rather, our results indicate that being accredited crowds in intrinsic motivation. This is especially the case when GPs perceive accreditation as an instrument for quality improvement. External interventions can therefore, at least in some cases, foster intrinsic motivation of health care professionals.