Serotonergic modulation of acetylcholine (ACh) release after neuron-specific increase of the expression of 5-HT(1B) receptors by gene transfer was studied in vitro and in vivo. The increased expression of the 5-HT(1B) receptor in vitro was induced by treating rat primary fetal septal cell cultures for 3 days with a viral vector inducing the expression of green fluorescent protein (GFP) vector alone, or, in addition, of 5-HT(1B) receptors (HA1B/GFP vector). The transfection resulted in a high number of GFP-positive cells, part of which being immunopositive for choline acetyltransferase. In HA1B/GFP-cultures (vs. GFP-cultures), electrically evoked ACh release was significantly more sensitive to the inhibitory action of the 5-HT(1B) agonist CP-93,129. Increased expression of the 5-HT(1B) receptor in vivo was induced by stereotaxic injections of the vectors into the rat septal region. Three days later, electrically evoked release of ACh in hippocampal slices of HA1B/GFP-treated rats was lower than in their GFP-treated counterparts, showing a higher inhibitory efficacy of endogenous 5-HT on cholinergic terminals after transfection. Moreover, CP-93,129 had a higher inhibitory potency. In conclusion, the HA1B/GFP vector reveals a useful tool to induce a targeted increase of 5-HT(1B) heteroreceptors on cholinergic neurons in selected CNS regions, which provides interesting perspectives for functional approaches at more integrated levels.