The effects of modafinil on glutamatergic and GABAergic transmission in the rat medial preoptic area (MPA) and posterior hypothalamus (PH), are analysed. Modafinil (30-300 mg/kg) increased glutamate and decreased GABA levels in the MPA and PH. Local perfusion with the GABAA agonist muscimol (10 microM), reduced, while the GABAA antagonist bicuculline (1 microM and 10 microM) increased glutamate levels. The modafinil (100 mg/kg)-induced increase of glutamate levels was antagonized by local perfusion with bicuculline (1 microM). When glutamate levels were increased by the local perfusion with the glutamate uptake inhibitor L-trans-PDC (0.5 mM), modafinil produced an additional enhancement of glutamate levels. Modafinil (1-33 microM) failed to affect [3H]glutamate uptake in hypothalamic synaptosomes and slices. These findings show that modafinil increases glutamate and decreases GABA levels in MPA and PH. The evidence that bicuculline counteracts the modafinil-induced increase of glutamate levels strengthens the evidence for an inhibitory GABA/glutamate interaction in the above regions controlling the sleep-wakefulness cycle.