Single neurones in the central nucleus of the ferret inferior colliculus (ICC) were studied using extracellular recording. Responses to pure tone stimuli were analyzed to assess the frequency organization of the nucleus, the sensitivity and tuning properties of neurones and the effects of binaural sound presentation. Excitatory tuning curves had the characteristic shape found for neurones in the auditory systems of other species. Many ferret neurones were inhibited by stimulus frequencies on either side of the range producing excitatory responses. Sharpness of excitatory tuning was found to be comparable with that reported for the cat. Neurones having best frequencies in the range 4-15 kHz showed the greatest sensitivity. All electrode penetrations revealed a dorsal-to-ventral progression of neurones of increasing best frequency. Some neurones were classified according to the predominant type of input from each ear. Over 80% of these units were binaurally influenced. Nearly all cells received an excitatory input from the contralateral ear and about one-third of those units were excited by ipsilateral stimulation. The remainder showed either a simple inhibitory ipsilateral input or ipsilateral inhibition that was manifested as a suppression of the excitatory, contralateral response. These experiments demonstrate that the ferret may be a satisfactory alternative to other carnivores for studies of the auditory system.