The fine structure of the2 f1−f2acoustic distortion product (ADP) was measured in humans with different primary level(L1/L2)and frequency(f2/f1,f2>f1)ratios. The(L1/L2)ratio was varied under two conditions. In the first conditionL1was fixed at 50 dB SPL whileL2was varied from 30 to 75 dB SPL in 5-dB steps. An upward frequency shift was observed in the ADP fine structure asL2was increased. In the second condition,L2was fixed at 50 dB SPL andL1varied, and a downward frequency shift was observed. These opposing frequency shifts are predicted by a vector-sum model [Sun et al., J. Acoust., Soc. Am.96, 2166–2174, 2175–2183 (1994)] and support the hypothesis that the ADP fine structure largely reflects place features of the area of overlap of the primary traveling waves. The mechanisms underlying the shifts in fine structure were further investigated by using three primaryf2/f1ratios: 1.11, 1.2, and 1.33. An orderly difference in the rate of fine-structure shift with level was observed as a function off2/f1ratio, with the largest rate of shift associated with the smallest frequency ratio. This observation, along with the fact that downward frequency shift (withL1varied) is always at a larger rate than the upward shift (withL2varied), suggests that ADP levels and fine structure are strongly influenced by the nonlinear compression present in the mechanics of the basilar membrane in the region of overlap between the primary traveling waves.