Single units in the central nucleus of the inferior colliculus (ICC) of barbiturate-anaesthetized cats were studied using pure-tone, best-frequency stimulation presented in the free field. At low stimulus intensities almost all neurones responded most strongly to stimuli positioned along the acoustical axis of the pinna contralateral to the recording electrode and there was little or no response to stimuli positioned in the ipsilateral hemifield. Four major classes of spatial response were distinguished when tones of moderate to high intensity were used. The simplest response (24% of the sample) to increasing intensity consisted of a monotonic increase in discharge level at all effective speaker positions and an expansion of the area of space from which a stimulus influenced the response (receptive field). A second class (21%) of units had a nonmonotonic increase in discharge level and an expanding receptive field with increasing intensity. Neither of these classes showed evidence of influence from the ipsilateral ear. The third class (26%) developed, at higher intensities, a second excitatory response region in the ipsilateral hemifield. The fourth class (20%) had receptive fields with fixed medial borders, irrespective of intensity. The third and fourth classes of units were thought to be binaurally influenced and to be sensitive to interaural phase and intensity differences, respectively.