Acoustic distortion has been measured from the ear canals of guinea pigs treated with the aminoglycoside antibiotic gentamicin. The correlation between pre- and post-treatment levels of distortion can be related to the extent of outer hair cell (OHC) pathology induced by gentamicin. All animals that were treated and show functional changes also show changes in OHC morphology. Changes in distortion properties are evident before clear changes in surface hair cell morphology are seen. In the early stages of intoxication, thin sections of the organ of Corti are necessary to reveal the underlying structural changes accompanying functional impairment. Where OHCs have been affected by gentamicin treatment, the greatest change in 2f1-f2 level is for distortion generated by widely spaced stimuli. With closely spaced stimuli, 2f1-f2 may be unchanged in level, but higher order distortion components are depressed. Functional evidence of mild OHC disruption is only revealed by stimulus levels at or below 60 dB SPL. With higher sound levels, apparently normal levels of distortion can be generated by ears in which there is clear evidence of OHC pathology. In a single, untreated animal with an extensive basal turn lesion, some 2f1-f2 distortion was generated when the stimulus frequency region lacked OHCs.