Employing a conditioning technique (in which the animal is required to shift position in order to avoid shock) and a beat-frequency oscillator in connection with an attenuator and crystal loud-speaker for the production of pure tones, the auditory threshold of rats was explored at frequencies of 1,000, 2,000, 4,000, 8,000, and 14,000 cycles/sec. (cps). 9 [male] rats were used, and threshold measured in terms of the lowest energy input (at each frequency) for which the animals made responses 50% of the time. Similar determinations were made for 9 human subjects (8 [male][male] and 1 [female]). Interest centered in relative, rather than absolute thresholds, and the results corroborate earlier findings that the rat ear is more sensitive at higher frequencies (above about 10,000 cps in the present study) than is man''s. Evidence was obtained that the rat hears well at 40,000 cps. The implications of the findings for studies of audiogenic seizures are briefly discussed.