Hearing relies on faithful synaptic transmission at the ribbon synapse of cochlear inner hair cells (IHCs). Previous studies have indicated that several vesicles are released in synchrony at this synapse, but the underlying presynaptic mechanism remained unclear. Here, we investigated whether IHC exocytosis uses parallel but statistically independent or dependent (cumulative or compound exocytosis) vesicle release. We measured exocytic IHC capacitance changes (deltaCm) and analysed the trial-to-trial fluctuations in order to estimate the apparent size of the quantal fusion event. We performed long lasting perforated-patch recordings stimulating mature and immature IHCs up to 200times. Short depolarizations were applied to preferentially recruit the readily releasable pool (RRP) of synaptic vesicles.We arrived at a quite narrow confidence interval of the apparent size of the quantal fusion event in IHCs before (121aF, CI95%: 27-213 aF) and after (90 aF, CI95%: 0-181 aF) the onset of hearing. Comparison to our vesicle capacitance estimates derived from electron microscopical measurements of vesicle diameter (50 aF) in mature IHCs suggests that the majority of fusion events in mature IHCs(~70%) was mediated by statistically independent fusion of synaptic vesicles. The remainder possibly reflected cumulative or compound fusion events involving a low number of synaptic vesicles.