Most criteria for establishing GABA as an inhibitory neurotransmitter in the central nucleus of inferior colliculus (ICc) have been satisfied, but the role of GABA in acoustic coding in ICc is not established. The present study examined this issue by evaluating the effects of iontophoretic application of agents that alter activity at GABA receptors on potential forms of acoustically-evoked inhibition in ICc neurons. Application of the GABAA antagonist, bicuculline, selectively blocked the firing reduction at high intensities observed during non-monotonic rate-intensity functions in ICc neurons. Binaural inhibition was selectively blocked by bicuculline and increased by nipecotic acid. Application of GABA, nipecotic acid (GABA uptake inhibitor) and a benzodiazepine (flurazepam), which enhances the action of GABA, increased the duration and intensity of ipsilateral inhibition and response pause, while bicuculline blocked these acoustically-evoked inhibitory events. Offset inhibition was increased by nipecotic acid application and reduced by bicuculline with the appearance of an offset peak. The present data support an important role for GABA as a neurotransmitter, mediating, in part, non-monotonicity, binaural inhibition, response pause and offset inhibition in ICc neurons. Alterations of these GABA-mediated inhibitory phenomena may occur in auditory dysfunctions observed with aging and audiogenic seizures.