Addictive properties of drugs of misuse are generally considered to be mediated by an increased release of dopamine (DA) in the ventral striatum. However, recent experiments indicated an implication of α1b-adrenergic receptors in behavioural responses to psychostimulants and opiates. We show now that DA release induced in the ventral striatum by morphine (20 mg/kg) is completely blocked by prazosin (1 mg/kg), an α1-adrenergic antagonist. However, morphine-induced increases in DA release in the ventral striatum were found to be similar in mice deleted for the α1b-adrenergic receptor (α1b-AR KO) and in wild-type (WT) mice, suggesting the presence of a compensatory mechanism. This acute morphine-evoked DA release was completely blocked in α1b-AR KO mice by SR46349B (1 mg/kg), a 5-HT2A antagonist. SR46349B also completely blocked, in α1b-AR KO mice, the locomotor response and the development of behavioural sensitization to morphine (20 mg/kg) and d-amphetamine (2 mg/kg). Accordingly, the concomitant blockade of 5-HT2A and α1b-adrenergic receptors in WT mice entirely blocked acute locomotor responses but also the development of behavioural sensitization to morphine, d-amphetamine or cocaine (10 mg/kg). We observed, nevertheless, that inhibitory effects of each antagonist on locomotor responses to morphine or d-amphetamine were more than additive (160%) in naïve WT mice but not in those sensitized to either drug. Because of these latter data and the possible compensation by 5-HT2A receptors for the genetic deletion of α1b-adrenergic receptors, we postulate the existence of a functional link between these receptors, which vanishes during the development of behavioural sensitization.