Masked thresholds were measured with an operant tracking method at 4 frequencies between 1000 and 8000 cps and at 3-5 levels of wide-band noise. As in humans and cats, masked thresholds in the rat increased linearly with noise level. Critical ratios were calculated from the masked thresholds and found to be 10 or 11 dB, [decibels] greater than those in man and 5 to 6 dB greater than those in cats. Greenwood''s function [DELTA] fCb=10ax+b, relating critical bandwidths to position on the basilar membrane, was shown to fit reasonably well the data of this experiment. The constant relation shown by Fletcher between critical ratios and frequency DL''s, [differential limen''s] previously found to apply to man and to cats, appears to apply to rats also.