Rats were divided into 2 groups according to their absolute auditory thresholds, as measured by a psychophysical method of limits using an operant conditioning technique. The procedure required a water-deprived rat to bar press repeatedly until a 3 s pure tone (3 kHz) came on. VR-15 schedule was used to determine the frequency of occurrence of the tone. Only within 4 s after the tone onset could the rat get water reinforcement by sticking its nose into the dipper area. The descending method was used with tone intensities ranging from 120 to 40 dB, in 2 dB steps. Imipramine was then administered 3 times at 24-h intervals (40, 80 and 160 mg/kg, p.o., respectively), and each rat was tested at 4, 8, and 12 h after each administration. Rats with high initial thresholds showed a decrease in thresholds with the lower dose of imipramine, and those with low initial thresholds an increase with the higher dose. An analysis of variance of these data showed significant effects of the type of subject (in terms of the level of initial thresholds), dose and interval conditions, and their interaction. It was demonstrated that the lower the initial sensitivity the greater the increase in sensitivity by imipramine.