The responses of units in the inferior colliculus of the urethane-anaesthetized rat were recorded extracellularly. They responded to sinusoidal AM and FM tones with a modulation of their spike discharge usually at the same, or occasionally at twice, the modulation rate of the stimulus. The modulation depth of the response initially increased with the modulation depth of the stimulus, but usually saturated or decreased at higher stimulus depths. The units showed a bandpass tuning to stimulus modulation rate which was independent of modulation depth and, in all cases, the most effective modulation rate was below 120 Hz. The modulated response to temporally varying stimuli could not be predicted from the pure tone discharge patterns or, in some cases, the unit's mean firing rate to modulated tones; temporally varying stimuli gave temporally varying responses. When compared with the data available from units at other levels in the auditory system, the results indicate a trend in which units at successively higher levels in the pathway respond most effectively to progressively lower rates of modulation.