Previous reports have suggested that IDPN may be ototoxic (Wolff et al., 1977; Crofton and Knight, 1991). The purpose of this research was to investigate the ototoxicity of IDPN using behavioral, physiological and morphological approaches. Three groups of adult rats were exposed to IDPN (0-400 mg/kg/day) for three consecutive days. In the first group, at 9-10 weeks post-exposure, thresholds for hearing of 5.3- and 38-kHz filtered clicks were measured electrophysiologically and brainstem auditory evoked responses (BAERs) were also recorded to a suprathreshold broadband click stimulus. A second set of animals was tested at 9 weeks for behavioral hearing thresholds (0.5- to 40-kHz tones) and at 11-12 weeks post-exposure for BAER thresholds (5- to 80-kHz filtered clicks). A third group of animals was exposed (as above), and killed at 12-14 weeks post-exposure for histological assessment. Kanamycin sulfate was used as a positive control for high-frequency selective hearing loss. Surface preparations of the organ of Corti were prepared in order to assess hair cells, and mid-modiolar sections of the cochlea were used to examine Rosenthal's canal and the stria vascularis. Functional data demonstrate a broad-spectrum hearing loss ranging from 0.5 kHz (30 dB deficit) to 80 kHz (40 dB deficit), as compared to a hearing deficit in kanamycin-exposed animals that was only apparent at frequencies greater than 5 kHz. Surface preparations revealed IDPN-induced hair cell loss in all turns of the organ of Corti, with a basal-to-apical gradient (more damage in the basal turns) at the lower dosages. At higher dosages there was complete destruction of the organ of Corti. There was also a dosage-related loss of spiral ganglion cells in all turns of the cochlea, again with a basal-to-apical gradient at the lower dosages. These data demonstrate that IDPN exposure in the rat results in extensive hearing loss and loss of neural structures in the cochlea.