The development of cochlear function was studied in 81 rat pups by recording the cochlear microphonic (CM) and the compound action potential (AP) from the round window in response to tone bursts and filtered clicks of various frequencies. The first electrophysiological response was the CM which could be obtained from 8- to 9-day-old animals. The AP first appeared at 11-12 days. As development progressed, cochlear potentials showed systematic changes in response parameters: the amplitude and threshold sensitivity of both CM and AP increased progressively with age as N1 latency decreased. All the parameters were within the adult range by the 15th day for the CM and by the 4th week for the AP. The results are discussed in relation to the developmental changes in the middle ear and to the morphological maturation of the organ of Corti.