In order to study carefully the configuration of response areas of auditory neurons in the medial geniculate body (MGB), we have examined an interaction between two tones known as "two-tone inhibition". This entails suppression of response activity to one tone by a second tone which is selected from a restricted range of frequencies and intensities. Single units were extracellulary recorded from non-paralyzed, lightly anaesthetized (pentobarbital, nitrous oxygen) adult cats. For each unit the discharge pattern, response latency, response area (RA), characteristic frequency (CF) and response-intensity functions were determined by binaural stimulation with pure sinusoids (0.1-25.6 kHz separated by Œ octave steps; stimulus duration 200 ms). During two-tone stimulation those tone bursts were delivered at fixed amplitude (usually 20 dB suprathreshold at CF) and a second tone of selected frequency and variable amplitude was presented simultaneously. Recording sites were defined electrophysiologically and localization was confirmed histologically.