Overshadowing can play an important role in conditioning with compound exteroceptive stimuli. Drug discrimination experiments have been carried out to examine overshadowing when mixtures of drugs serve as compound interoceptive stimuli. Three groups of rats were trained in a two-bar operant procedure with a tandem schedule of food reinforcement (n = 8). All rats were trained to discriminate (-)-nicotine (0.32mg/kg s.c.) from saline, but in two groups of animals midazolam (0.1 or 0.2mg/kg s.c.) was co-administered with the nicotine to generate a compound stimulus. Dose-response curves were determined with nicotine and midazolam in each group. In rats trained with nicotine alone, there was a steep dose-response curve for the discriminative stimulus effect of nicotine. The presence of the smaller dose of midazolam in the training stimulus clearly attenuated, and the larger dose prevented, the appearance of the discriminative effect of nicotine, whereas there was a concomitant increase in the discriminative response to midazolam. These results suggest that midazolam overshadowed the response to nicotine in a dose-related manner. In rats trained with nicotine alone, the same doses of midazolam had no effect on the discriminative response established to the nicotine stimulus, indicating the absence of pharmacological antagonism. The results illustrate how conditioning factors may provide a behavioural mechanism for interactions between abused drugs.