The sensitivity of sound perception depends on the frequency - each detected at a specific tonotopic location in the cochlea. Here, we investigated whether the properties of the afferent hair cell synapses could contribute to this phenomenon. We found that the number of hair cell synapses had a maximum in the cochlear region that transmits sounds with highest sensitivity (10-24 kHz). Confocal microscopy of the organ of Corti following immunostaining for the ribbon marker RIBEYE and for GluR2 and 3 as postsynaptic markers was performed to estimate the number of afferent synaptic contacts as colocalized spots of pre- and postsynaptic immunofluorescence. We then investigated the presynaptic function of inner hair cells at different positions along the apical turn of the cochlea. Probing exocytosis by measurements of cell capacitance increments after brief depolarizations, we found that hair cells located 300 mm from the apex ? thus responsive to very low frequency sounds ? released 44% less transmitter than cells located at ~1400 mm from the apex. This functional finding corresponded to a 31% difference in the number of morpho-logically identified afferent synapses betwen these locations. Interestingly, size, charge and kinetics of the calcium current as well as ribbon size did not vary with the tonotopic position of the hair cells.