The current article argues that an important component of the research agenda for Relational Frame Theory will involve studying the functional relations that obtain between environmental events and the physiological activity that takes place inside the brain and central nervous system, with a particular focus on human language and cognition. In support of this view, five separate experiments are outlined. The first three experiments replicate and extend previous research reported by Hayes and Bisset (1998). Specifically, the research, using both reaction time and neurophysiological measures, supports the argument that there is a clear functional overlap between semantic and derived stimulus relations. Specifically, an evoked potential waveform typically associated with semantic processing (N400) is shown to be sensitive to equivalence versus non-equivalence relations. Experiments 4 and 5 indicate that these reaction time and evoked potential effects are not restricted to traditional lexical decision tasks, but can also be observed using the implicit association test. Furthermore, preliminary evidence suggests that evoked potentials might constitute a more sensitive measure of derived stimulus relations than response time. The results obtained across all five experiments support the view that the study of derived stimulus relations, combined with some of the procedures and measures of cognitive psychology and cognitive neuroscience, may provide an important inroad into the experimental analysis of semantic relations in human language.