In vivo microdialysis and single-cell extracellular recordings were used to assess the involvement of serotonin(2A) (5-HT(2A)) and serotonin(2C/2B) (5-HT(2C/2B)) receptors in the effects induced by amphetamine and morphine on dopaminergic (DA) activity within the mesoaccumbal and nigrostriatal pathways. The increase in DA release induced by amphetamine (2 mg/kg i.p.) in the nucleus accumbens and striatum was significantly reduced by the selective 5-HT(2A) antagonist SR 46349B (0.5 mg/kg s.c.), but not affected by the 5-HT(2C/2B) antagonist SB 206553 (5 mg/kg i.p.). In contrast, the enhancement of accumbal and striatal DA output induced by morphine (2.5 mg/kg s.c.), while insensitive to SR 46349B, was significantly increased by SB 206553. Furthermore, morphine (0.1-10 mg/kg i.v.)-induced increase in DA neuron firing rate in both the ventral tegmental area and the substantia nigra pars compacta was unaffected by SR 46349B (0.1 mg/kg i.v.) but significantly potentiated by SB 206553 (0.1 mg/kg i.v.). These results show that 5-HT(2A) and 5-HT(2C) receptors regulate specifically the activation of midbrain DA neurons induced by amphetamine and morphine, respectively. This differential contribution may be related to the specific mechanism of action of the drug considered and to the neuronal circuitry involved in their effect on DA neurons. Furthermore, these results suggest that 5-HT(2C) receptors selectively modulate the impulse flow-dependent release of DA.