Auditory-evoked potentials, originating from the brain stem and the forebrain, were studied in 30 unanesthetized kittens during the first 3 months of postnatal life, and from a smaller set of animals before and after surgical exposure of their tympanic membranes. In intact animals, responses to 135-dB peak SPL clicks were first reliably discernible on the seventh postnatal day; when stimuli were presented directly to the exposed tympanic membrane, responses were observed several days earlier. Responses progressed through three stages during maturation: an early period of gross insensitivity during which responses are evoked only by high-intensity stimuli and whose response thresholds remain essentially constant (week 1); a middle period characterized by rapid acquisition of sensitivity to near-adult values (week 2); and a late period during which adult thresholds and latencies are acquired. A sequential stage model of threshold maturation is proposed, in which thresholds decline linearly during stage two and exponentially during stage three. It is hypothesized that mechanical reorganization of the cochlea during the first 2 to 3 postnatal weeks and development of the stria vascularis are primarily responsible for the linear stage, and that neural factors primarily underlie the exponential stage and account for the gradual acquisition of adult thresholds. Rates of maturation for brain stem responses are frequency dependent, with responses to high frequencies achieving adult thresholds earlier than those to low frequencies.