In experiment 1, masking patterns were obtained with a tonal masker that was sinusoidally amplitude modulated (SAM) at a rate of 8 Hz and a depth(m)of 1.0. The signal was centered at a masker peak or masker valley. Masker frequency(fm)was 750, 1350, or 2430 Hz, and signal frequency(fs)ranged from 0.8 to1.62 fm.Thresholds were generally higher for a signal in a masker peak than in a masker valley. The magnitude of this peak-to-valley (PV) difference was governed byfs/fm,rather than byfs,and was largest forfs>fm.The PV differences were smallest at the lowestfm,at least whenfs>fm.In experiment 2, growth-of-masking functions were measured (fm=1350 Hz,fs=1.44fm). The masker was modulated at a depth(m)of 1.0, 0.75, or 0.50. These thresholds were compared with those obtained with an unmodulated masker in forward or simultaneous masking. The comparisons suggest that thresholds for a signal at a peak of an 8-Hz SAM masker are due to simultaneous masking, while those in a valley are due primarily to forward masking whenm=1.0or simultaneous masking whenm=0.75or 0.50. For these masker depths, the PV difference first increased but then decreased as masker level increased from 60 to 90 dB SPL. This was a consequence of the slope of the masking function for peak placement changing from a value greater than 2.0 to a value of 1.0 at the highest signal levels (an effect that was also observed with the unmodulated simultaneous masker), a result that may be understood in terms of basilar membrane nonlinearity.