Tone-on-tone masking patterns were measured in the chinchilla using the auditory evoked response from the inferior colliculus. The masker was a 2 kHz continuous tone presented at one of five levels from 20 to 70 dB SPL. The evoked response was elicited with tone bursts presented over a wide range of frequencies. Masking was defined as the difference between evoked response thresholds measured in the presence of the masker versus quiet. At low masker levels, the masking profile was symmetrical and centered around 2 kHz. However, as masker level increased, masking spread toward the high frequencies. Furthermore, at masker levels above 50 dB SPL, a low-threshold notch occurred above the masker. The masking profiles obtained with the evoked response are in good agreement with those obtained psychophysically.