According to relational frame theory Cfunc stimuli select which stimulus properties are transformed via derived stimulus relations. To date there has been no demonstration of the selective action of Cfunc control. We provide an analysis of the requirements for such a demonstration, and describe the results from four experiments employing a paradigm consistent with these requirements. We employed a paradigm based on virtual car races. The paradigm had two components: i) a sample racecar screen which showed the performance of a sample racecar, and used experimentally engineered symbols to communicate how the performance of each real racecar would compare with that of the sample racecar, and ii) a car race screen showing other racecars race. Two symbols were established as Crels for the relations of same and different, and two symbols were established as Cfuncs for the functional properties of speed and direction. The results from these experiments demonstrate Cfunc stimuli can select which functions transform via derived stimulus relations, a central component of relational frame theory. The study has implications for the study of relational responding in complex settings and for applied work aimed at refining repertoires of relational responding.