The effects of fluoxetine were studied on cloned K+ channel Kv1.4 stably expressed in Chinese hamster ovary (CHO) cells using the whole-cell configuration of the patch-clamp technique. Extracellular application of various concentrations of fluoxetine inhibited the amplitude of the peak current of Kv1.4 and accelerated its inactivation time course in a concentration-dependent manner. Thus, fluoxetine decreased Kv1.4 (the integral of the outward current) in a concentration-dependent manner; the IC50 was 33.1 ± 2.5 μM. The inhibitory effect of fluoxetine was time-dependent. The apparent association (k) and dissociation (l) rate constants measured at +40 mV were 3.5 ± 0.7 μM-1s-1 and 132.5 ± 13.3 s-1, respectively. The Kd (= l/k) was 37.9 μM, which was close to the value obtained from the concentration-response curve. The block produced by fluoxetine increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. The fluoxetine block was constant at more depolarized potentials, suggesting that the block by fluoxetine was not voltage dependent. Our data indicate that fluoxetine blocks Kv1.4 channels by preferentially binding to open state.