The discriminative effects produced by morphine in the rat were evaluated using a two-choice, discrete trial avoidance task. Stimulus control of behavior was attained with a dose of morphine one-third to one-tenth of that used in previous studies. Morphine produced dose-related discriminative effects over a 100-fold dose range. The stimulus control produced by the discriminative effects of morphine met the following criteria for classification as a specific narcotic effect: 1) oxymorphone, levorphanol, methadone and meperidine, narcotic analgesics from diverse chemical families, also produced dose-related morphine-like discriminative effects; 2) dextrorphan and thebaine, compounds structurally related to the narcotics but lacking narcotic activity, failed to produce morphine-like discriminative effects; 3) effects were blocked by the narcotic antagonist naloxone; and 4) tolerance to the discriminative effects developed upon the repeated administration of morphine and cross-tolerance extended to methadone. The discriminative effects produced by morphine were further characterized by evaluating the capacity of prototypes of other classes of psychoactive drugs to produce morphine-like discriminative effects. Profadol and pentazocine, euphorogenic analgesics with mixed agonist and narcotic antagonist properties, produced dose-related morphine-like discriminative effects whereas cyclazocine, a dysphorogenic analgesic with mixed agonist and narcotic-antagonist properties, did not. In addition, the nonopioid psychoactive drugs d-amphetamine, pentobarbital and chlorpromazine also failed to produce morphine-like discriminative effects. Thus, morphine-like discriminative effects were produced uniquely by the narcotic analgesics and euophorogenic analgesics with mixed agonist and narcotic antagonist properties. These results suggest that the component of action of morphine that enables it to function as a discriminative stimulus in the rat is analogous to the component of action of morphine responsible for producing subjective effects in man.