The perceived duration of 1-kHz pure tones with increasing or decreasing intensity profiles was measured. The ratio between the down- and up-ramp durations at equal subjective durations was examined as a function of the sound duration (50, 100, 200, 500, 1000, 2000 ms). At 50 and 100 ms, the ratio was constant and equaled about 1.7, then it logarithmically decreased from 100 to 1000 ms to reach a constant value of 1 at 1 and 2 s. The different mechanisms proposed in the literature to explain the perceived duration asymmetry between up-ramp and down-ramp were discussed in the light of the dependence of this ratio on duration.