To understand the role of dopamine in the mediation of opiate's analgesic and reinforcing activity, the influence of pharmacologically-induced increase (apomorphine in a high dose) and decrease (haloperidol and apomorphine in a low "presynaptic" dose) of dopamine neurotransmission on morphine-induced depression of vocalization and movement reactivity was investigated in freely behaving rats. Strong enhancement of morphine-induced vocalization reactivity depression and a weakening of movement reactivity depression typical to concomitant pharmacological increase of dopamine neurotransmission may represent the enhancement of its reinforcing and analgesic properties and inhibition of its depressive/sedative activity. Enhancement of MOR-induced depression of movement and vocalization reactivity characteristic of drug-induced decrease of dopamine neurotransmission may be the consequence of the blockade of opiate's reinforcing and stimulatory activity and summation of depressive/sedative actions typical to these drugs administered alone.