The representation of frequency was mapped in the primary auditory cortex (AI) of C57BL/6J (C57) mice during young adulthood (1.5-2 months) when hearing is optimal, and at 3, 6, and 12 months of age, a period during which progressive, high frequency, sensorineural hearing loss occurs in this strain. Maps were also obtained from CBA/CaJ mice which retain good hearing as they age. In AI of young adult C57 mice and CBA mice, characteristic frequencies (CFs) of multiple-unit clusters were easily identified with extracellular recordings, and a general tonotopic organization was observed from dorsal (high frequency) to ventral and caudal (low frequency). In individual cases there appeared to be deviations from the above tonotopic organization, despite the fact that inbred mice are genetically invariant. As progressive loss of high frequency sensitivity ensued peripherally, a substantially increased representation of middle frequencies was observed in AI. There was no apparent change in the surface area of the auditory cortex despite the elimination of high frequencies, and virtually the entire auditory cortex became devoted to the middle frequencies (especially 10-13 kHz) for which sensitivity remained high. Similar age-related changes were not observed in normal-hearing CBA mice. These findings indicate that plasticity in the representation of frequency in AI is associated with high frequency hearing loss in C57 mice.