Inferior colliculus (IC) slices from guinea pigs were prepared by cutting the IC parasagittally and incubating the obtained slices in the standard medium. Postsynaptic field potentials (PSFP) were recorded in the superficial layer of the pericentral nucleus (PN) by stimulating the deep layer of the PN. GABA (gamma-aminobutyric acid) and its agonists (muscimol and baclofen) and antagonist (bicuculline) were applied in the perfusion medium. GABA (1-12.5 mM), muscimol (1-200 microM) and (-)-baclofen (0.1-20 microM) reduced the amplitude of the PSFP in a dose-dependent manner. Bicuculline (1-10 microM) enhanced the amplitude of the PSFP and evoked the epileptiform burst discharge (EBD). The incubation of IC slices with methoxypyridoxin (100 microM) and 3-mercaptopropionic acid (100 microM) for 2 h reduced the GABA level of the slice to about 50% of the initial value, at which point a distinct EBD appeared. The application of GABA (5 mM) to the medium in this condition blocked the EBD completely. The removal of chloride from the medium as well as the application of furosemide (100 microM) enhanced the amplitude of the PSFP and also elicited the EBD; GABA application in this case did not block the EBD. These results indicate that GABA plays an inhibitory role in the IC, and that the reduction of GABA and the blockade of GABA function cause the appearance of EBD in the IC.