Duration is a salient feature of acoustic signals including speech. Duration tuning was first reported in flogs and later in echolocating bats. More recently, duration tuning has been reported in non-echolocating mammals and appears to be a fundamental encoding mechanism throughout the animal kingdom. However, the duration tuning reported in these non-echolocating mammals appears to be much weaker than that in the previous studies on bats. In contrast to this finding, our recent study reported that duration tuning in the IC in guinea pigs appeared to be strong when it was measured using an appropriate temporal window. With such a temporal window, duration tuning was found to be compatible with that of echo-locating bats. In the present report, we further demonstrate that duration tuning in the IC of this species is established by interaction between excitation and GABAergic inhibition. In addition to overall increase in responsiveness, application of bicuculline (B1C), a GABA-A receptor antagonist, was found to significantly reduce or eliminate duration selectivity in 44 out of the 67 neurons that showed clear duration tuning from a sample of 340 neurons.