Extracellular spike activity was recorded from single units in the medial geniculate body (MGB) of nitrous oxide anaesthetized cats. The responses of 291 units to tone bursts at the characteristic frequency (CF) were studied as a function of stimulus intensity, covering a range from 10 to 100 dB SPL. The proportion of MGB units characterized by a monotonic or a non-monotonic discharge rate--intensity function was 26% and 74%, respectively. In addition, changes of response latency as a function of tone levels were demonstrated to be either monotonic (38% of units) or non-monotonic (62% of units). One third Of MGB units showed a change of response pattern with increasing intensities, in similar proportion towards either prevailing excitatory or inhibitory components. The monotonic units tended to differ from non-monotonic ones in addition to their intensity function by showing shorter response latencies, a higher response probability to broad-band stimuli and simpler response patterns. The mean dynamic range of the monotonic unit population was 60 dB, with thresholds ranging from 10 to 90 dB SPL; most discharge rate--intensity functions did not saturate at sound levels of 100 dB SPL. In the population of non-monotonic units, the 'best' intensity, defined as th intensity giving the strongest response, ranged between 10 and 100 dB SPL. The present results suggest that the intensity could be signaled by the mean firing rate of a restricted population of monotonic units or place coded by the distribution of maximally activated non-monotonic units which are broadly tuned to different intensities.