Metkephamid effects on operant behavior.

Intravenous injections of high doses of metkephamid (20 and 40 mg/kg) decreased responding by pigeons under a multiple fixed-ratio, fixed-interval schedule of grain presentation. Naloxone antagonized in a dose-related manner the suppression of behavior produced by 40 mg/kg of metkephamid. Daily maintenance on large doses (30 and 60 mg/kg PO) of dlmethadone produced a slight shift of the dose-effect curve of metkephamid to the right. The data suggest that the behavioral effects of metkephamid are due to an action at a mu-opioid receptor.