In order to study the effects of efferent activity, olivocochlear efferents were stimulated with an electrode in the fourth ventricle at the decussation of the crossed olivocochlear bundle (midline-OCB stimulation) or with an electrode at the brainstem origin of medial efferents (MOC stimulation). Tuning curves, or similar measures of threshold, were obtained from auditory-nerve fibers in the presence or absence of efferent stimulation. Efferent stimulation raised the thresholds of fibers for tones at the characteristic frequency (CF) by an amount which varied with the spontaneous rate (SR) of the auditory-nerve fiber. On the average, high-SR fibers had the smallest threshold shifts, and low-SR fibers had the largest threshold shifts. The distribution of threshold shifts as a function of CF peaked at CFs of 3-8 kHz for high-SR and medium-SR fibers but appeared to peak at higher CFs for low-SR fibers. Within the high-SR or medium-SR groups, the fibers with the lowest thresholds had the largest threshold shifts. Efferent stimulation decreased the Q20 of the tuning curves from most fibers (i.e. it made the tuning curves wider), but increased the Q20 from some fibers with CFs below 2 kHz. For fibers with CFs above 4 kHz, efferent stimulation shifted the tuning-curve tails to higher sound levels by about 1 dB on the average. The qualitative patterns of the effects due to midline-OCB stimulation or to MOC stimulation were similar. The distribution of high-SR threshold shifts vs. CF appears to be displaced apically in the cochlea compared to the distribution of MOC endings on outer hair cells. This can be understood in terms of efferent activity depressing basilar membrane motion and affecting regions at, and apical to, the activated efferent synapses. To explain the low-SR threshold shifts, an additional way in which efferent activity inhibits responses appears to be required. The data are consistent with one function of the medial efferents being to raise the thresholds of auditory-nerve fibers and thereby adjust the effective range of the auditory system.