Six pigeons were studied to determine whether the brightness of a houselight interacted with the stimuli produced by methadone, and whether the nature of the interaction depended on the order of training of the two discriminations. Three pigeons were trained to peck the right key after methadone (2.0mg/kg) and the left key after saline, when the houselight was dim. The effects of a range of methadone doses (0.5, 1.0, 2.0mg/kg and saline) were tested. Three other pigeons were trained, in the absence of drug, to peck the right key when the houselight was dim and the left key when the houselight was bright. The effects of a range of houselight intensities were tested. Then, for both groups, right-key pecks were reinforced in the presence of methadone and the dim houselight, and left-key pecks were reinforced in the presence of saline and the bright houselight. Methadone doses were tested in the presence of both houselight brightnesses used in training. All pigeons pecked the methadone-appropriate key after high doses of methadone, regardless of houselight intensity. All pigeons trained to discriminate houselight brightness first, and one of the pigeons trained to discriminate methadone first, pecked according to the houselight condition when saline and lower doses were tested. In the other pigeon trained to discriminate methadone first, pecking was more related to drug dose. These data show that a drug stimulus can compete with external stimuli for behavioral control, that a drug stimulus can assume control over behavior originally controlled by external stimuli, and that discriminations based on external stimuli may be retained when saline or low doses of drug are administered.