Phase locking between probe and masker was used in a series of pure-tone masking experiments. The masker was a stationary sine wave of variable frequency; the probe a fixed-frequency tone burst. We have observed that for small frequency separation the masking behaves asymmetrically around the probe frequency. This asymmetry depends on intensity. For a 1-kHz probe at low stimulus levels there is a maximum masking effect at about 60 Hz above the probe frequency, whereas at high levels maximum masking is produced at a frequency definitely below the probe frequency. These results are discussed in relation to current neurophysiological and psychophysical data. For the high-level assymetry possible interpretations are suggested in terms of two changes in the excitation pattern of the basilar membrane, (a) a shift of the top and/or (b) a slope asymmetry, both increasing with level. The low-level asymmetry will be treated in a second paper [Vogten, J. Acoust. Soc. Am. 63, 1521-1528 (1978)].