The effects of forward masker duration on psychophysical measures of frequency selectivity were investigated in two experiments. In both experiments, masker duration was 50 or 400 ms, signal duration was 20 ms, and there was no delay between masker offset and signal onset. In the first experiment, growth‐of‐masking functions were measured for a masker whose frequency was below, at, or above the 1000‐Hz signal frequency. From those data, input filter patterns (IFPs) were plotted for masker levels from 40–90 dB SPL. In the second experiment, masking patterns (MPs) were measured for a 1000‐Hz masker presented at 50, 70, and 90 dB SPL. Both measures of frequency selectivity (IFPs and MPs) indicate that frequency selectivity is greater for the 400‐ms masker. These data suggest that there may be a sharpening of frequency selectivity with time at a stage prior to the adaptation observed in forward masking.