Three separate drug-discrimination experiments, using pigeon subjects, were designed to assess the modification of a morphine (MS) stimulus by d-amphetamine (AMPH). Experiment 1 examined the drug mixture in a two-choice discriminative procedure in three groups of subjects trained to one of three doses of MS (1.0, 3.2, or 10 mg/kg) vs. saline (SAL). Blockade of the MS stimulus by concomitant AMPH administration was training dose-dependent. Complete blockade was found in the high-MS (10 mg/kg) training dose (TD) group; partial blockade was found in the intermediate-TD (3.2 mg/kg) group; and complete generalization was found in the low-TD (1.0 mg/kg) group. Experiment 2 was designed to assess whether the MS-AMPH mixture resulted in the creation of a functionally new stimulus. With a three-choice drug-discrimination procedure, two new groups were trained to discriminate between 3.2 mg/kg MS, SAL, and 1.8 mg/kg AMPH. Once training criteria were met, subjects were divided into two groups and one group's MS training dose was faded up to 10 mg/kg. Data clearly indicated that MS-AMPH combinations did not produce novel stimuli. Experiment 3 was designed to assess the MS-AMPH combinations in a three-choice quantitative discrimination using 1.8 mg/kg MS vs. SAL vs. 10 mg/kg MS as discriminative stimuli. Data suggested that if masking does occur, it is a result of shifting the dose-response function to the left (i.e., to a nondiscriminable dose). We conclude that perceptual masking of drug stimuli can occur, but the phenomenon seems dependent on training dose and/or drug and behavioral history.