Difference limens for level [delta Ls (dB) = 20 log[p + delta p)/p), where p is the pressure] were measured as a function of duration for tones at 250, 500, and 8000 Hz. Stimulus durations ranged from 2 ms to 2 s, and the stimulus power was held constant. Rise and fall times were 1 ms. The interstimulus interval was 250 ms. At each frequency, three levels were tested: 85, 65, and approximately 40 dB SPL. An adaptive two-alternative forced-choice procedure with feedback was used. For three normal listeners, delta Ls decreased as duration increased, up to at least 2 s, except at 250 Hz. At 250 Hz, delta L stopped decreasing at durations between 0.5 and 1 s. In a double logarithmic plot of delta L versus duration, the rate of decrease is generally well fitted by a sloping line. The average slope is -0.28; it is steeper at high levels than at low levels. Because the average slope is shallower than the -0.5 slope predicted for an optimum detector, it may be that fast adaptation of auditory-nerve activity and/or memory effects interfere with level discrimination of long-duration tones. Finally, the delta Ls at 8 kHz decreased nonmonotonically with increasing level.