Modulation rate discrimination (Δfm) thresholds were obtained for sinusoidally amplitude modulated (SAM) pure tones. The carrier frequencies were 1000 and 4000 Hz and the modulation rates were 5, 10, 20, and 50 Hz. The Δfm thresholds of one carrier (the probe) were also obtained when the other carrier (the masker) was presented simultaneously. The masker was either SAM or was unmodulated. In all conditions the depth of SAM was 100%. This experiment was designed to determine how much the modulated masker interferes with the discrimination of modulation rate and how this interference changes as a function of the difference in modulation rate between the masker and probe. It has been shown previously [Yost and Sheft, J. Acoust. Soc. Am. Suppl. 1 83, S35 (1988)] that a modulated masker interfered significantly with a listener's ability to detect SAM of the probe when the masker and probe were modulated at the same rate. This modulated detection interference (MDI) was reduced when the masker was modulated at a different rate than the probe. These earlier results suggested that there might be auditory channels “tuned” to different rates of amplitude modulation. The present experiment was designed to further test the concept of AM tuned channels. [This research was supported by the NINCDS.]