ACh release from the rat frontal cortex was increased by both local, 0.1-1 muM, and systemic, 0.1-10 mug/kg, administration of the 5-HT3 receptor antagonist ondansetron, reaching a maximum peak of 143% over basal values. Bicuculline, 1-10 muM, and flumazenil, 5-10 mg/kg, antagonists at different sites of the GABA(A) receptor, also enhanced ACh release, with maximum effects of 85 and 124% above baseline, respectively. GABAA receptor antagonists potentiated the effect induced by ondansetron on ACh release, reaching a peak increase of 238% (with bicuculline) and 259% (with flumazenil) over basal levels. These results suggest an interaction of ondansetron with GABAergic neurons modulating ACh release in the rat frontal cortex in vivo. (C) 2002 Elsevier Science B.V. All rights reserved.