The binaural input to single neurones in the inferior colliculus of barbiturate- and ketamine-anaesthetised kittens was tested using pure tones. Stimuli were presented to each ear independently via sealed sound-delivery systems. Cells were classified according to the predominant type of input (excitatory or inhibitory) from each ear. There was no significant difference between age groups in the proportions of cells showing each type of binaural input. For all ages, cells excited by monaural stimulation of either ear (EE cells) tended to be of low best frequency (BF) and cells excited by monaural stimulation of one ear, but inhibited by simultaneous stimulation of the other ear (EI cells), had higher BFs. Units for which quantitative spike count data were obtained were also categorised according to their response pattern. Cells categorised as "onset", "pauser" and "burst-type" were found in approximately equal proportions in animals of all age groups. "Primary-like" units were not found in animals less than 20 days old. Unit discharge rate was significantly higher for adults than for any of the three kitten groups. These data suggest that (1) binaural connections are not subject to postnatal developmental alteration and (2) the development of excitatory responses after 20 days consists of an increase in the number of discharges rather than a change in the pattern of these discharges.