Rate-intensity functions (RIFs) were generated in response to characteristic frequency (CF) tones presented alone and in the presence of broadband noise for neurons in the central nucleus of the inferior colliculus (IC) of the anesthetized guinea pig. Seventy-six percent of the RIFs to CF tones were monotonic (some showing incomplete saturation), and 24% were nonmonotonic. The RIFs to continuous noise were more nonmonotonic than those to CF tones. In continuous or gated noise, the dynamic portion of the RIF to a tone was shifted to a higher tone level, with little change in the dynamic range. Above a threshold noise level, the shift was a linear function of noise level with slope 0.97. Little shift occurred when the noise was inversely gated with respect to the tone burst, suggesting that the underlying mechanism is suppression rather than adaptation. For 63% of units, the maximum discharge rate to a tone in low levels (less than 0-dB spectrum level) of noise (including inversely gated) was greater than to the tone alone. Although many of the effects of noise in the IC reflect peripheral mechanisms, they are supplemented by centrally based processes which enhance the detectability of tone intensity increments in the presence of noise.