Rats were exposed to ethyl benzene at 0, 300, 400 and 550 ppm for 8 hours/day for 5 consecutive days. Three to six weeks after the exposure, auditory function was tested by measuring compound action potentials (CAP) in the frequency range of 1-24 kHz and 2f1-f2 distortion product otoacoustic emissions (DPOAEs) in the frequency range of 4-22.6 kHz. In addition, outer hair cell (OHC) loss was quantified by histological examination. The lowest concentration ethyl benzene had no effect on any of the above measures. At 400 ppm, auditory thresholds were increased by 15 and 16 dB at 12 and 16 kHz, respectively, and at 550 ppm by 24, 31, and 22 dB at 8, 12, and 16 kHz, respectively. DPOAE amplitude growth with stimulus level was affected only after 550 ppm at 5.6, 8, and 11.3 kHz. OHC loss was found in two of the five examined locations in the cochlea. At 400 ppm, 25% OHC loss was found at the 11- and 21-kHz region. The highest concentration evoked 40% and 75% OHC loss at the 11- and 21-kHz location, respectively. Thus, the mid-frequency region of rats is affected after exposure to relatively low concentrations of ethyl benzene (400-550 ppm). These results indicate that ethyl benzene is one of the most potent ototoxic organic solvents known today.