The commissure of the inferior colliculus (CoIC) interconnects corresponding frequency-band laminae in the two inferior colliculi (ICs). Although the CoIC has been studied neurophysiologically in vitro, the effect of the CoIC on the responses of IC neurons to physiological stimuli has not been addressed. In this study, we injected the glutamate receptor blocker kynurenic acid into one IC while recording the frequency response areas (FRAs) of neurons in the other, to test the hypothesis that frequency response properties of IC neurons are influenced by commissural inputs from the contralateral IC. Following blockade of the commissure, 10 of 12 neurons tested exhibited an increase or a decrease in their FRAs. In most neurons (9/12) the response area changed in the same direction, irrespective of whether the neuron was stimulated monaurally (at the ear contralateral to the recorded IC) or binaurally. In one neuron, blockade of the CoIC resulted in an expansion of the response area under binaural stimulation and a contraction under monaural stimulation. In the remaining two units, no effect was observed. Changes in response areas that exceeded the criterion ranged between 17 and 80% of control values with monaural stimulation, and 35 and 77% with binaural stimulation. Area changes could also be accompanied by changes in spike rate and monotonicity. From our observation that FRAs contract following commissure block, we infer that the commissure contains excitatory fibres. The expansion of response areas in other cases, however, suggests that the commissure also contains inhibitory fibres, or that its effects are mediated by disynaptic as well as monosynaptic circuits. The small sample size precludes a definitive conclusion as to which effect predominates. We conclude that inputs from the contralateral IC projecting via the CoIC influence the spectral selectivity and response gain of neurons in the IC.