Duration is a biologically important feature of sound. Some neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus, are tuned to sound duration, but it is unclear at what level the tuning originates or what neural mechanisms are responsible for it. The application of antagonists of the inhibitory neurotransmitters gamma-aminobutyric acid or glycine to neurons in the inferior colliculus eliminated duration tuning. Whole-cell patch-clamp recordings of synaptic currents suggested that inhibition produces a temporal frame within which excitation can occur. A model is proposed in which duration tuning arises when an early, sustained inhibitory input interacts with a delayed, transient excitatory input.