This paper reports on a professional development activity (PD) for course leaders, which was designed within the European project Primas (2010-13, www.primas-project.eu). Primas aimed at the implementation of inquiry-based learning (IBL) in day-to-day teaching on a large scale. The focus is on the education of mathematics course leaders in Germany, who were all teachers. In order to support them in taking up their role and run courses on (IBL), a course leader seminar following design research was developed. The course aimed at course leaders' learning on two levels: (1) as mathematics teachers about IBL and (2) as course leaders about running PD on IBL. A small scale retrospective interview study gives insights into the needs the participants face when learning about IBL as teachers and when learning as future course leaders. It also illuminates in which way participants took up their role as a course leader. These insights suggest how to optimize the course following design research.