In Experiment 1, 3 college students were exposed to relational pretraining to establish the contextual functions of Same, Opposite, More Than, and Less Than in four arbitrary stimuli. Subjects were then trained on the matching-to-sample tasks A 1-81 and Y1-N1, in the presence of the More-Than contextual cue, A 1-82 and Y1-N2 in the presence of the Less-Than contextual cue, C1-D1 and E1 -D2 in the presence of the Same cue, and C1-D2 and E1-D1 in the presence of the Opposite cue. Test trials were subsequently administered to probe for the mutually entailed relations; Less-Than/81-A 1, Less-ThanlN1-Y1, More-Than/82-A1 , More-Than/N2-Y1 , Same/D1-C1, Same/D2-E1, Opposite/D2-C1, and Opposite/D1-E1. Response latencies to probes for derived Same/Opposite relations were significantly lower than those for derived More ThaniLess Than relations. Experiment 2 exposed 4 subjects to training across each of the four relations and used a novel stimulus set to test for reduced response latencies to the derived relations. Response latencies to More-ThaniLess-Than probes reduced significantly across the original to the novel stimulus set, whereas latencies to Same/Opposite probes were low across both stimulus sets.When human subjects are trained on a series of conditional discriminations, the stimuli involved may control responding in ways that are not readily predicted using traditional behavioral principles. For example, if choosing the arbitrary stimuli, B1 and C1, is reinforced on separate trials, given a further stimulus, A 1, then, the presentation of B1 or C1 may reliably occasion the choosing of A1 (symmetry) without reinforcement. Furthermore, the presentation of B1 may control choosing C1, and the presentation of C1 may control choosing B1 (combined symmetry and transitivity), again without reinforcement. These effects are collectively referred to as stimulus equivalence and the stimuli A 1, B 1, and C1 are said to participate in an equivalence relation (Barnes, 1994;Barnes & Holmes, 1991;Hayes, 1991;Sidman, 1971Sidman, , 1990).Requests for reprints should be addressed to Bryan Roche, Department of Psychology, National University of Ireland, Maynooth , Co. Kildare, Ireland. The authors thank Michael Clayton and an anonymous reviewer for their helpful comments. The first three authors say farewell to Cork, where so many ideas were born .