The behavioral effects of naltrexone and quaternary naltrexone were evaluated in two groups of pigeons. One group responded under a fixed-ratio schedule of food reinforcement and was trained to discriminate between 3.2 mg/kg morphine (i.m.) and saline. Drug-appropriate responding occurred in a dose-related manner to morphine, given intramuscularly and intraventricularly. When administered intraventricularly morphine was 50 times more potent as a discriminative stimulus and 50-100 times more potent at suppressing responding. Naltrexone, given intraventricularly and intramuscularly, attenuated the discriminative stimulus effects of morphine. Quaternary naltrexone was more potent at suppressing responding when administered intraventricularly but it failed to attenuate the discriminative stimulus effects of morphine. A second group of pigeons, responding under a variable-interval schedule of food reinforcement, was treated with 100.0 mg/kg/day of morphine. Naltrexone and quaternary naltrexone suppressed responding by both routes of administration. Naltrexone was approximately equipotent when given intramuscularly or intraventricularly, and the doses that suppressed responding were 50-500 times smaller than doses required to suppress responding in untreated pigeons. Although quaternary naltrexone was 1800 times more potent when given intraventricularly, the doses necessary to suppress responding by each route were the same as doses required in untreated pigeons. These results extend the conditions under which a quaternary derivative of naltrexone failed to display antagonist activity in the pigeon. The utility of this compound for characterizing central and peripheral mechanisms of action has not been established in different species and testing conditions and, therefore, appears to be appropriate only under conditions in which it is evaluated after systemic, as well as central, administration.