The goal of the mammalian neurophysiological research is to describe quantitatively the response of single auditory neurons to complex sounds in the chinchilla auditory system. Extracellular single unit recording is the primary method used to study neuronal responses. The responses of single neurons to complex sounds are described in terms of average firing rate and temporal discharge properties. Neurophysiological experiments have focused on the responses of neurons in the cochlear nucleus. The cochlear nucleus receives its entire afferent input from the auditory nerve and is the first stage in the auditory pathway where neural processing can occur. The cochlear nucleus is comprised of a variety of prinipcal cells that differ in morphology and physiology. Consequently, for any given sound, there are multiple neural representations of that sound in the outputs of the cochlear nucleus. The goal of these physiological experiments is to discover the neural correlates that can account for the behavioral performance as described from human and chinchilla psychophysical experiments. Current experiments focus on the responses of cochlear nucleus neurons to amplitude modulated sounds and to iterated rippled noises. These are the same complex stimuli that are used in the human and chinchilla psychophysical experiments.