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Two experiments examined the effects of morphine on the key pecking of pigeons maintained by fixed-interval schedules of food delivery. The first experiment was a systematic replication of Laties and Weiss (1966). Pecking was maintained by a multiple fixed-interval fixed-interval “clock” schedule with programmed interreinforcer intervals of 5 min. Evidence for disruption of patterning was found during the final stimulus in the clock sequence, which is inconsistent with previous suggestions that temporal patterning is unchanged in the presence of external clock stimuli. Experiment 2 examined the effects of morphine on response patterning during a fixed-interval 1-min schedule with interpolated temporal discrimination trials. Morphine disrupted patterning during fixed-interval trials and accuracy during temporal discrimination trials. Accuracy was equally disrupted following short and long sample durations. These effects are inconsistent with interpretations of the disruption of response patterning as a selective “overestimation” of time. The effects of morphine may be related to the effects of more conventional external stimuli on response patterning. Specifically, it is suggested that drugs and other stimuli may change response patterning by changing inhibitory and excitatory stimulus-generalization gradients within fixed intervals.