Fragile X syndrome (FXS) is characterized by hypersensitivity to sensory stimuli, including environmental sounds. We compared the auditory brainstem response (ABR) recorded in mice lacking the gene ( ) for fragile X mental retardation protein (FMRP) with that in wild-type animals. We found that ABR wave I, which represents input from the auditory nerve, is reduced in animals, but only at high sound levels. In contrast, wave IV, which represents the activity of auditory brainstem nuclei is enhanced at all sound levels, suggesting that loss of FMRP alters the central processing of auditory signals. Current-clamp recordings of neurons in the medial nucleus of the trapezoid body in the auditory brainstem revealed that, in contrast to neurons from wild-type animals, sustained depolarization triggers repetitive firing rather than a single action potential. In voltage-clamp recordings, K currents that activate at positive potentials ("high-threshold" K currents), which are required for high-frequency firing and are carried primarily by Kv3.1 channels, are elevated in mice, while K currents that activate near the resting potential and inhibit repetitive firing are reduced. We therefore tested the effects of AUT2 ((4-({5-(4R)-4-ethyl-2,5-dioxo-1-imidazolidinyl-2-pyridinyl}oxy)-2-(1-methylethyl) benzonitrile, a compound that modulates Kv3.1 channels. AUT2 reduced the high-threshold K current and increased the low-threshold K currents in neurons from animals by shifting the activation of the high-threshold current to more negative potentials. This reduced the firing rate and, , restored wave IV of the ABR. Our results from animals of both sexes suggest that the modulation of the Kv3.1 channel may have potential for the treatment of sensory hypersensitivity in patients with FXS. mRNA encoding the Kv3.1 potassium channel was one of the first described targets of the fragile X mental retardation protein (FMRP). Fragile X syndrome is caused by loss of FMRP and, in humans and mice, causes hypersensitivity to auditory stimuli. We found that components of the auditory brain response (ABR) corresponding to auditory brainstem activity are enhanced in mice lacking FMRP. This is accompanied by hyperexcitability and altered potassium currents in auditory brainstem neurons. Treatment with a drug that alters the voltage dependence of Kv3.1 channels normalizes the imbalance of potassium currents, as well as ABR responses , suggesting that such compounds may be effective in treating some symptoms of fragile X syndrome.