Single neurones in the central nucleus of the inferior colliculus (ICC) of barbiturate-anesthetized cats were examined using free-field, pure-tone stimuli of low intensity at the neurones' best frequency. Receptive field size was inversely correlated with best frequency. Almost all neurones were maximally excited by stimulus positions in the hemifield contralateral to the recording electrode, irrespective of their best frequency. Simultaneous cochlear microphonic recording revealed that the neurones' best excitatory area was also the spatial region associated with maximum amplification by the contralateral outer ear. This amplification resulted in extremely low (less than -20 dB SPL in some neurones) best frequency thresholds. Response patterns were found not to vary markedly with speaker position. The results suggest that most ICC neurones are more sensitive to stimulation of the contralateral ear than to stimulation of the ipsilateral ear.