The psoralen, a furocoumarin derivative, on the cloned neuronal rat Kv3.1 channels stably expressed in Chinese hamster ovary cells was investigated using the whole-cell patch-clamp technique. Psoralen reduced Kv3.1 whole-cell currents in a reversible concentration-dependent manner, with an $IC_{50}$ value and a Hill coefficient of $2.3;{pm};0.03;{mu}M$ and $0.9;{pm};0.08$, respectively. Psoralen accelerated the decay rate of inactivation of Kv3.1 currents without modifying the kinetics of current activation. The psoralen-induced inhibition of Kv3.1 channels was voltage-dependent, with a steep increase over the voltage range of channel opening. However, the inhibition exhibited voltage independence over the voltage range in which channels are fully activated. Psoralen slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of psoralen, were superimposed. Inhibition of Kv3.1 by psoralen was use-dependent at a frequency of 1 Hz. The present results suggest that psoralen acts on Kv3.1 currents as an open-channel blocker.