Many animals use duration to help them identify the source and meaning of a sound. Duration sensitive neurons have been found in the auditory midbrain of mammals and amphibians, where their selectivity appears to correspond to the lengths of species-specific vocalizations. In this study, single neurons in the rat inferior colliculus (IC) were tested for sensitivity to sound duration. About half (54%) of the units sampled showed some form of duration selectivity. The majority of these (76%) were long pass neurons that responded to sounds exceeding some duration threshold (range: 5 to 60 ms). Band pass neurons, which only responded to a restricted range of durations, made up 13% of duration sensitive neurons (best durations: 15 to 120 ms). Other units displayed short pass (2%), or mixed (9%) response patterns. The majority of duration sensitive neurons were localized outside the central nucleus of the IC, especially in the dorsal cortex, where over half of the neurons sampled had long pass selectivity for duration. Band pass duration tuned neurons were only found outside the central nucleus. Characteristics of duration sensitive neurons in the rat support the idea that this filtering arises through an interaction of excitatory and inhibitory inputs that converge in the IC. Band pass neurons typically responded at sound offset, suggesting that their tuning is created through the same mechanisms that have been described in echolocating bats. The finding that the first spike latencies of all long pass neurons were longer than the shortest duration to which they responded supports the idea that they receive transient inhibition prior to, or simultaneously with a sustained excitatory input. The ranges of selectivity in rat IC neurons are within the range of durations of rat vocalizations. These data suggest that a population of neurons in the rat IC have evolved to transmit information about behaviorally relevant sound durations using mechanisms that are common to all mammals, with an emphasis on long pass tuning characteristics.