Twelve subjects were trained to select one of two stimuli from a pair (the B pair) when presented with one of two stimuli from another pair (the A pair), thus establishing two AB relations, Al-BI and A2-B2. In a similar fashion, additional stimuli were used to establish BC, CD, and DE relations. Trials used to train all relations occurred in each session. Once performances were established, probe trials were introduced that tested for the emergence of untrained relations (e.g., BI-DI or Al-El).These emergent relations were categorized according to nodal distance (i.e., the number of stimuli across which transitivity would have to hold in order for the relation to emerge). For example, a test for A2-C2 crosses one node (B2), whereas a test for Al-El crosses three nodes (Bl, Cl, and Dl).Only 2 of the subjects formed equivalence classes. The evocation of class-appropriate responding by each emergent-relation probe was an inverse function of nodal distance for all 12 subjects. In addition, performance on the originally trained relations was disrupted by the introduction of probes. The 2 subjects who exhibited equivalence classes were then trained to make different numbers of key presses in the presence of each of the four A and E stimuli. In a response-transfer test, the B, C, and D stimuli evoked the responses trained to the A and E stimuli in the same equivalence class. Likelihood of class-appropriate responses was an inverse function of nodal distance, and this pattern persisted across testing. Reaction times in the transfer test were an inverted U-shaped function of nodal distance. Because training of the baseline relations occurred concurrently and the B, C, and D stimuli were presented an equal number of times before the transfer test, the test performances illustrate effects of nodal distance that were not confounded by order or amount of experience with the stimuli. The results imply that ordered, sequential exposure to individual stimulus relations may facilitate the development of equivalence classes and that the relatedness of stimuli within an equivalence class is a relatively permanent inverse function of nodal distance. Willson-Morris, 1985). It has also been supported by more recent studies that systematically measured the choice performances (Dube, Green, & Serna, 1993;Fields, Adams, Newman, & Verhave, 1992;Fields, Adams, Verhave, & Newman, 1990;Kennedy, 1991;Kennedy, Itkonen, & Lindquist, 1994) and the reaction times (Bentall, Dickins, & Fox, 1993;Dickins, Bentall, & Smith, 1993;Wulfert & Hayes, 1988) occasioned by tests of emergent relations. The most recent support was provided by Fields, Adams, Verhave, and Newman (1993), who showed that nodal distance influenced transfer of responding among members of an equivalence class.In all of these studies, however, the baseline relations were trained in a serial manner. As a result, nodal distance was confounded with the order of stimulus introduction during training and most likely with the number of times each stimulus was presented in