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Four experiments were conducted to test whether transfer of stimulus control could be obtained between two drug facilitators, a drug and non-drug facilitator, and two non-drug facilitators. In the first experiment, the presence of methadone signaled which of two key lights would be followed by grain. In the absence of methadone, the contingencies were reversed. Concurrently, the presence of phencyclidine (PCP) signaled which of two additional key lights would be followed by grain. In the absence PCP, the contingencies were also reversed. During transfer tests in which each conditioned stimulus (CS) was presented following administration of each drug facilitator, only the CS that was appropriate to the current drug condition elicited responding. Thus, transfer of facilitative functions across drug stimuli was not obtained. In the second experiment, one of the drug stimuli was removed and, in its place, the presence or absence of a tone served as the facilitator. Tests of transfer between the tone and the remaining drug facilitator found little or no evidence of transfer of facilitative function. A third experiment attempted to replicate Experiment 1 using considerably less training. For the most part, the results from the transfer tests replicated the findings of Experiment 1, that is very little evidence of transfer was obtained. For one bird, however, methadone occasioned pecking to both the methadone-CS and the PCP-CS but not the saline-CS. Thus, asymmetrical transfer was obtained for 1 pigeon. A fourth experiment was conducted using two external stimuli as facilitators (a tone and a flashing houselight). Transfer was obtained in 2 of the 3 pigeons tested. That transfer was obtained between two non-drug facilitators (Experiment 4) but not between two drug facilitators (Experiments 1 & 3) and between a drug and non-drug facilitator (Experiment 2) suggests that the facilitative function of a drug stimulus is specific to the CSs used during training (a CS-dependent mechanism), whereas the facilitative function of a non-drug stimulus is not specific to the CSs used during training (a CS-independent mechanism).