The threshold for detection of pure tones in broadband noise was determined for three cats using an auditory reaction time procedure. Critical ratio is defined as the ratio of the signal power at the masked threshold for detection to the spectrum level of the noise. Critical ratios were obtained for 250-, 500-Hz, and 1-, 2-, 4-, 8-, and 16-kHz tones over a wide range of noise intensities. Results indicate that critical ratios increase with the frequency of the tone stimulus. At frequencies below 4 kHz, critical ratios remain constant at moderate and high noise intensities. For frequencies above 4 kHz, critical ratios increase as the level of the masking noise is raised from moderate to high levels. The difference between low- and high-frequency behavior of the level dependence of critical ratios is considered in terms of two possible mechanisms: (1) different mechanisms may be involved in the encoding of low- and high-frequency information by the nervous system or (2) the difference in level dependence may be due to attenuation by the action of the middle ear muscles at high sound levels.