Male Wistar rats were exposed to a free choice between water and a morphine-sucrose solution. Following stabilization of baseline levels of consumption of morphine, the animals were injected for 5 consecutive days with either FLA-57 (45 or 60 mg/kg i.p.), a non-toxic dopamine-beta-hydroxylase inhibitor or its vehicle. The FLA-57 treated animals significantly attenuated their preference for morphine during the injection and post-injection periods although there were no significant differences related to the dosages used. These treatments produced a concomitant reduction in central norepinephrine levels suggesting that norepinephrine may be involved in the mediation of the reinforcing properties of morphine consumed by laboratory rats. The possibility of common neural mechanisms regulating the pharmacological actions of both morphine and ethanol are discussed.