In pigeons (Columba livia; N = 5) trained to discriminate the mu (mu) opioid butorphanol from saline, various mu (morphine, fentanyl) but not kappa (kappa; bremazocine, U50,488, U69,593) opioids produced at least 80% drug-appropriate responding. Subsequently, pigeons were retrained to discriminate the kappa opioid bremazocine from saline; in this condition, both mu and kappa opioids produced at least 80% drug-appropriate responding, naloxone was more potent as an antagonist of the butorphanol stimulus, and chronic bremazocine treatment selectively produced tolerance to the bremazocine stimulus. Although butorphanol can function as a kappa antagonist, the effects produced by combinations of butorphanol and bremazocine during training with bremazocine were generally effect-additive. The findings indicate that the mechanisms underlying the stimulus effects of mu opioids are not altered by subsequent training with a kappa opioid. Conversely, a history of discrimination training with a mu opioid does not alter the discriminative control produced by kappa opioids during subsequent training with a kappa opioid.