Toluene is a widely used organic solvent causing loss of auditory sensitivity in rats and presumably in humans. Also, the hearing loss in humans occupationally exposed to noise has been reported to be aggravated by simultaneous exposure to solvents. The aim of the present investigation was to study the effects of toluene, alone or in combination with other factors, on auditory sensitivity. The influence of acoustic trauma and of acetyl salicylic acid (ASA) on toluene induced auditory sensitivity loss in the rat, as well as the influence on age-related auditory sensitivity loss in two genotypes of mice were studied. An attempt was also made to identify cochlear structures damaged by toluene. Rats or mice were exposed to toluene by inhalation during 1 or 2 weeks. Rats were exposed to noise before or after the toluene exposure, or exposed to ASA by gavage simultaneously with toluene. The effect on auditory sensitivity was measured by frequency specific auditory brainstem response (ABR) between 1.6 and 20 kHz. In the investigations concerning the site of the toluene induced impairment, distortion product otoacoustic emissions (DPOEs) and morphological techniques were used. A permanent loss of auditory sensitivity, as measured by ABR, with the maximal threshold shifts between 6.3 and 12.5 kHz, was found in rats after exposure to toluene. In addition, lowered amplitudes of the DPOEs were recorded, implying that the outer hair cells (OHCs) were affected by toluene. A morphological study confirmed that toluene mainly affects the OHCs. A potentiated loss of auditory sensitivity was seen when toluene had preceded the noise exposure. When the exposure sequence was reversed, i.e. noise preceded toluene, an additive effect was observed. ASA, itself known to cause a reversible auditory impairment, was found to enhance the loss of auditory sensitivity caused by simultaneous toluene exposure. Finally, toluene exposure accelerated the age-related hearing loss in C57BL/6J mice. In conclusion, toluene causes a characteristic mid-frequency auditory damage. The OHCs are particularly susceptible to toluene exposure. The auditory sensitivity loss induced by toluene can be enhanced by ASA and by ensuing noise exposure. Also, toluene exposure can influence the rate of progress of age-related hereditary loss of auditory sensitivity.