It was previously shown that the acute administration of adenosine elicits an antidepressant-like effect in the mouse forced swimming test (FST) by a mechanism dependent on the inhibition of the L-arginine-nitric oxide (NO)-guanylate cyclase pathway. Taken into account that the stimulation of this pathway is associated with the activation of K(+) channels, this study investigated the involvement of different types of K(+) channels in the effect of adenosine in the FST. Intracerebroventricular treatment of mice with tetraethylammonium (TEA, a non-specific inhibitor of K(+) channels, 25 pg/site), glibenclamide (an ATP-sensitive K(+) channel inhibitor, 0.5 pg/site), charybdotoxin (a large- and intermediate-conductance calcium-activated K(+) channel inhibitor, 25 pg/site) or apamin (a small-conductance calcium-activated K(+) channel inhibitor, 10 pg/site) was able to potentiate the action of subeffective doses of adenosine (1 mg/kg, i.p.) and fluoxetine (a serotonin reuptake inhibitor, 10 mg/kg, i.p.). Furthermore, the administration of adenosine or fluoxetine and the K(+) channel inhibitors, alone or in combination, did not affect the ambulatory behavior. Moreover, the reduction in the immobility time elicited by active doses of adenosine (10 mg/kg, i.p.) or fluoxetine (32 mg/kg, i.p.) in the FST was prevented by the pretreatment of mice with cromakalim (a K(+) channel opener, 10 microg/site, i.c.v.), without affecting the locomotion in an open-field. Together these results indicate that the modulatory effects of adenosine and fluoxetine on neuronal excitability, via inhibition of K(+) channels, may represent the final pathway of their antidepressant-like effects in the FST.