The development of cochlear responses in the mouse was investigated, recording from the round window. Positive summating potentials (SP) could be detected as early as 7 days after birth in some individuals, the first signs of negative SP occurred in mice aged 10 days, and compound action potentials were first detected at 11 days of age. These early responses were obtained with relatively low frequency stimuli (usually 6 kHz). All waveforms recorded from 20 day old mice could be interpreted as simple additions of positive and negative SP and compound action potentials, each with different amplitudes and latencies. Positive SP showed both fast and slow components. Our observations are consistent with the positive SP with only a fast component arising from the basal turn inner hair cells, the positive SP with fast and slow components being generated by both inner and outer hair cells in the basal turn, and the negative SP seen at low frequencies of stimulation originating from depolarisation of hair cells in the apical turn. Summating potentials may thus be useful for investigating inner and outer hair cell function separately in abnormal cochleas in which it is not known which cell type primarily is affected.