In rats vertically implanted with concentric dialysis probes in the medial prefrontal cortex and in the medial nucleus accumbens, morphine, ethanol and nicotine failed to modify extracellular dopamine in the medial prefrontal cortex at doses that were fully effective in raising extracellular dopamine in the nucleus accumbens. Conversely, the aversive/anxiogenic drugs picrotoxin, pentylenetetrazol and FG 7142, administered at subconvulsant doses, increased extracellular dopamine in the medial prefrontal cortex but failed to do so in the nucleus accumbens. Systemic administration of low doses of the 5HT3 antagonist ICS 205930, previously reported to prevent the increase of extracellular dopamine in the nucleus accumbens elicited by morphine, nicotine, ethanol and haloperidol (Carboni et al. 1989) as well as by stress (Imperato et al. 1990), also prevented the increase of extracellular dopamine elicited in the prefrontal cortex by anxiogenic drugs. Therefore, mesocortical and mesolimbic dopamine neurons show clear-cut differences in the reactivity to drugs of abuse and to aversive drugs but are both modulated by a facilitatory serotonergic input mediated by 5HT3 receptors.