Recent data obtained in our laboratory suggest that the slope of the psychometric function in forward masking can vary widely across conditions, with shallower slopes occurring in conditions with higher signal levels at threshold. The goal of the present study was to separate possible effects of masker frequency, masker level, and signal delay from the effect of signal level per se. Thresholds and psychometric functions were obtained for three normal-hearing listeners using a 2IFC adaptive task. The signal was a 4-kHz, 10-ms tone. The on-frequency masker was a 200-ms tone presented at 30- to 90-dB SPL in 10-dB steps. The off-frequency masker was a 2.4-kHz, 200-ms tone presented at 60- to 90-dB SPL in 10-dB steps. Signal delays were 0, 10, or 30 ms. As expected, forward masking grew more rapidly with increases in masker level in the off-frequency than the on-frequency conditions. Signal delay had little independent effect on the slopes of the psychometric functions. The effects of masker level and signal level were similar for on- and off-frequency masker conditions in most cases. The results will be discussed in the context of a model that incorporates peripheral nonlinearity and internal noise. [Work supported by NIH.]