Binaural interaction plays an important role in shaping response properties of central auditory neurons. Using single unit recording and iontophoresis, we examined frequency tuning curves (FTCs), interaural intensity difference (IID) curves, and rate-intensity functions of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus, under closed system or free field stimulation conditions. We isolated 46 EI (excitation-inhibition), 24 EO (monaural excitation) and 6 EE (excitation-excitation) neurons. Inhibitory FTCs of EI neurons plotted under ipsilateral sound stimulation fell within (n=10, 22%), partly overlapped (n=26, 56%), or almost entirely encompassed (n=10, 22%) excitatory FTCs plotted by contralateral sound stimulation. The discharge rate of EI neurons was a sigmoid function of IID. The peak discharge rate occurred at IIDs at which contralateral sound stimulation was stronger than ipsilateral sound stimulation. Application of bicuculline, an antagonist for gamma-aminobutyric acid A receptors, raised the IID curves and broadened the excitatory FTCs but partly or completely abolished the ipsilateral inhibitory FTCs. For EE neurons, excitatory FTCs and rate-intensity functions plotted by contralateral sound stimulation were always broader and higher than those plotted by ipsilateral sound stimulation. The sharpness of FTCs of EI neurons was significantly greater at ipsilateral 30 degrees than at 30 degrees contralateral. This direction-dependent frequency tuning was effectively abolished by occlusion of the ipsilateral ear. Possible mechanisms underlying these observations are discussed.