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Off-frequency masking of the second formant by energy at the first formant has been shown to influence both identification and discrimination of the second formant in normal-hearing and hearing-impaired listeners. While both excitatory spread and two-tone suppression have been implicated in this simultaneous masking, their relative contribution has been shown to depend on both the level of the masker and the frequency separation between the probe and the masker. Off-frequency masking effects were evaluated in 10 normal-hearing human adults using the frequency-following response (FFR) to two two-tone approximations of vowel stimuli (/a/ and /u/). In the first experiment, the masking effect of F1 on F2 was evaluated by attenuating the level of F1 relative to a fixed F2 level. In the second experiment, the masking effect was evaluated by increasing the frequency separation between F1 and F2 using F2 frequency as the variable. Results revealed that both attenuation of the F1 level, and increasing the frequency separation between F1 and F2 increased the magnitude of the FFR component at F2. These results are consistent with a release from off-frequency masking. Given that the results presented here are for high signal and masker levels and for relatively smaller frequency separation between the masker and the probe, it is possible that both suppression and excitatory spread contributed to the masking effects observed in our data.