Key-peck responses of pigeons on one of two keys were reinforced intermittently under a multiple fixed-interval schedule. In one component of the schedule, a houselight provided general illumination of the experimental chamber, and responses on a red key produced food according to a 5-min fixed-interval schedule. In the alternate component, the houselight was off and responses on an amber key produced food according to the 5-min fixed-interval schedule. Each response randomly (P = .5) alternated the positions of the key colors (right or left) and the two components (houselight on or off) alternated in a mixed sequence. Thus, there were two sources of discriminative control over responding: 1) the presence or absence of the houselight and 2) the key colors. Stimulus control was assessed by comparing the relative frequencies of red-key responses in the presence and absence of the houselight. Pentobarbital decreased stimulus control of responding at intermediate (3.0-10.0 mg/kg) doses that did not appreciably alter average rates of responding. Whereas d-amphetamine decreased stimulus control only at high doses (3.0-5.6 mg/kg) that also substantially decreased response rates. The results of subsequent studies suggested that the two drugs primarily affected stimulus control exerted by the colors of the response keys. Furthermore, the effects on stimulus control produced by the two drugs were modified by a number of environmental conditions, in particular those that alter the degree of stimulus control existing before drug administration.