Experiments on the effects of drugs on behavior maintained by temporal-discrimination procedures have led to discrepant results. Recent experiments suggest that the effects of drugs may differ depending on whether the subject is timing some aspect of its own behavior or some other stimulus. The present experiment used a multiple-schedule procedure composed of a subject-produced and experimenter-imposed component. In the subject-produced component, pigeons categorized the duration of their most recently emitted interresponse time. In the experimenter-imposed component, pigeons categorized the duration of a key light. Morphine generally produced underestimation of time during the subject-produced component, a result in agreement with other recent experiments. Morphine had no systematic effects on accuracy during the experimenter-imposed component. These results are discussed in terms of procedural interactions and a morphine-induced disruption of stimulus control.