Two cynomolgous macaques categorized six colors into two groups of three after conditional discrimination training (zero-delay symbolic match-to-sample). The procedures resulted in the establishment of relations among the elements of each set-relations that were not specifically trained and that can be characterized by the properties of reflexivity, symmetry, and transitivity. Each set of colors was related to a characteristic pattern of responding: One response pattern involved temporal duration (press and hold the response keys); the second response pattern entailed repeated pressing and releasing of the response keys (fixed ratio 8). Six combinations of two colors were trained, three combinations from each set. After discriminative performance stabilized for each monkey, they were tested with 10 additional color combinations, all of which differed from the training combinations. The conditional relations established between test combinations can be characterized as stimulus equivalence. The training procedures were analogous to the procedure of using category names, and have implications for understanding the function of language in the formation of equivalence classes.

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Three experiments conducted in an automated ten-compartment chamber recorded collateral activities of rats reinforced for lever pressing on differential-reinforcement-of-lowrate schedules. In Experiment 1, the rate of lever pressing increased when stimulus support for collateral activities was removed, thus confirming earlier findings. However, there were no temporal or sequential patterns of collateral activities that predicted operant responding.In Experiment 2, the rate of lever pressing increased only if (a) access to all stimulus support for collateral activities was simultaneously prevented, and (b) the rat was forced to remain in the presence of the lever and food tray. The availability of any of the stimuli related to collateral activity was sufficient to keep lever-pressing rates from increasing. Experiment 3 examined collateral activities under a signaled differential-reinforcementof-low-rate schedule. Preventing access to stimuli supporting collateral activities had little effect on stable lever pressing when the signal was maintained. When the signal was removed, collateral activities continued, but lever-pressing rates increased in three of the four rats and rates of food presentation declined in all rats. Hypotheses that collateral activities have (a) a timekeeping or discriminative function, or (b) directly inhibit operant responding were not supported. The results suggest that collateral activities may facilitate operant responding by simply removing the subject from the presence of reinforcementrelated stimuli.Key words: DRL, fixed-time, collateral, lever press, competition, concurrent, ratsExperimental manipulation of collateral activities affects the rate of operant responding under the control of differential-reinforcement-of-low-rate (DRL) schedules. Preventing the occurrence of established collateral activities increases the rate of operant responding, and providing stimuli for the occurrence of collateral activities generally reduces the rate of operant responding.There are four hypotheses regarding the inverse relationship between collateral activities and operant responding. First, collateral activities serve as the timekeeping mechanism and become discriminative for the operant response (e.g

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