This study aims to compare experimentally the effect of gentamicin administration on the cochleae of guinea pigs of different ages. While it is frequently cited that age may represent a predisposing factor towards aminoglycoside ototoxicity in clinical reports, little experimental work has been carried out on this topic. Two groups of albino guinea pigs, either 4 weeks or 24 weeks old, were used. Animals were injected subcutaneously for 10 consecutive days with gentamicin at a dose of either 50 or 125 mg/kg/day, or with an equivalent volume of saline to act as controls. The survival period after the last injection was either 4 or 12 weeks. On being killed, both bullae were removed and fixed in osmium tetroxide, and prepared of phase-contrast microscopy by the surface preparation technique. The entire hair cell population of one cochlea from each animal was recorded onto cochleograms, enabling detailed graphical and statistical analysis. Both age groups display extremely similar patterns of outer and inner hair cell loss at the higher dose; the lower dose of gentamicin was not ototoxic in this study. Hair cell loss is predominantly basal, and tends to be maximally concentrated in two areas of the base of the cochlea. While mortality is much higher in the older animals, there is no difference in the response of the two age groups to the ototoxic effects of gentamicin.