1. The activity of single units in the inferior colliculus of unanesthetized monkeys was recorded during performance in an auditory reaction time task. Stimulus intensity and frequency were varied. 2. Spontaneous rate of unit discharge varied from 0 to 78.2 discharges per second, with a mean of 14.7 discharges/sec. 3. Both broadly and narrowly tuned units were encountered in the central nucleus of the inferior colliculus. The temporal discharge pattern of most units varied with changes in stimulus frequency; onset bursts and/or sustained discharge suppression dominated the unit discharge at the edges of receptive fields. 4. Half of the units examined at several intensity levels displayed nonmonotonic relationships between evoked discharge rate and stimulus intensity, with most nonmonotonic units showing a distinct "best intensity". The temporal response pattern of almost all units varied with changes in stimulus intensity, with onset bursts and discharge suppression increasing in occurrence with increasing intensity. 5. Units recorded in the external nucleus of the inferior colliculus displayed spontaneous rates which were similar to those of central nucleus units, and were affected by variation in stimulus intensity in the same fashion. However, the average initial latency of such units to intense stimuli was no longer than the latency of central nucleus units. 6. Variations in unit discharge with changes in stimulus frequency and intensity are consistent with an interaction of excitatory and inhibitory inputs with different initial latencies, dynamic ranges and receptive fields. In particular, our data suggest that inhibitory inputs have longer initial latencies and higher thresholds. Inhibition is stronger at the edges of a unit's receptive field, and dominates at high frequencies in units with low characteristic frequency. 7. Our data are not consistent with previous reports that single units in the unanesthetized animal display uniformly monotonic intensity functions and uniformly broad frequency responses.