Single units of the auditory nerve are maximally sensitive to a given frequency, the so-called characteristic frequency (CF). Tuning curves (functions relating the "threshold" for such units to the frequency of a sinusoidal stimulus) have inspired psychophysical measurements of masked thresholds in humans. At low levels, the results bear a striking resemblance to neural tuning curves, suggesting that a small, homogeneous group of units determine the so-called psychophysical tuning curves. This paper describes physiological measurements modeled after the psychophysical forward-masking paradigm. A masking tone was used to reduce the response in auditory-nerve fibers elicited by a probe tone that followed the masker. The level of the masking tone was varied to maintain a criterion level of responding to the probe. Plotting level of the masker as a function of its frequency yields a fiber-masking function. For low probe-tone levels, fiber-masking functions of low CF fibers and the tips of functions from high CF fibers were similar in shape to tuning curves obtained with single tone bursts. The fiber-masking function depends upon the response criterion and upon the level, duration, and frequency of the probe. Varying the level of the probe can also alter the shape of the fiber-masking function. These results have implications for interpreting psychophysical masking functions.