C57BL/6J (C57) mice demonstrate progressive age-related hearing loss during the first year of life, whereas CBA/J (CBA) mice lose little sensitivity through 18 months of age. The acoustic startle response (ASR) was measured in these strains to determine behavioral correlates of aging with and without presbycusis. The stimuli were tone pips (10-ms duration, 1-ms rise-fall) with frequencies of 4, 8, 12, 16, or 24 kHz at intensities of 70, 80, 90, or 100 dB SPL. ASR "thresholds" (the minimum SPL required to elicit ASRs more than 50% of the time) increased with age, and startle amplitudes became smaller in both strains. However, the changes in these startle parameters were much more pronounced in C57 mice, with middle to high frequencies (12-24 kHz) severely affected. The startle latencies at and above ASR "threshold" increased with age in C57 mice, but did not change in CBA mice. The CBA data indicate that aging, per se, has little effect on ASR parameters; the C57 data show that hearing loss is a cogent factor. However, ASR parameters of C57 mice are altered to a greater extent than expected, on the basis of the elevations of absolute sensory thresholds, particularly for middle frequencies (12-16 kHz). Both peripheral and central mechanisms are proposed to account for the discrepancy.