Background In auditory, vestibular and lateral line systems, ribbon synapses allow hair cells to drive high rates of sustained synaptic transmission (Goutman and Glowatzki 2007; Pichler and Lagnado 2019) and to maintain the exquisite temporal precision of sensory information. The kinetic properties of vesicle release and replenishment at these synapses have mainly been studied by measuring changes in membrane capacitance during hair cell depolarization using patch clamp, which bypasses the normal process of mechanotransduction. The zebrafish is becoming an important model for the investigation of hair cell development and function (Ricci et al. 2013; Olt et al. 2014; Pichler and Lagnado 2019), with the advantage to investigate both pre- and postsynaptic responses and the activity of afferent neurons in an intact organism in vivo. In this study, we investigated the release properties of hair cell ribbon synapses and sensory adaptation in vivo using a combination of extracellular recording and fluorescence imaging while displacing the mechanoelectrical transducer apparatus.   Methods Work was licensed by the UK Home Office and approved by the University of Sheffield Ethical Review Committee. Zebrafish larvae from the following strains were used: Tg(brn3c:Gal4), Tg(UAS:iGluSnFR), Tg(UAS:GCaMP7a) and Tg(nbt:GCaMP3). Zebrafish larvae (<5.2 days post fertilisation) were anaesthetised in MS-222 (0.01%) prior to injection of α-bungarotoxin (125 μM) into the heart to suppress movement. Recordings from the ganglion cell body were performed using loose-patch voltage-clamp as previously described (Trapani et al. 2011). Mechanical displacement of the cupula was achieved by using a piezoelectric fluid jet. iGluSnFR and Ca2+ signals were recorded using a two-photon laser-scanning microscope based on a mode-locked laser system operating at 925 nm.   Results Sustained displacement of the neuromast cupula elicited a transient increase in hair cell synaptic release at the stimulus onset, which quickly adapted to a lower, sustained level. The release time course was consistent with the recruitment of two pools of synaptic vesicles with different release kinetics. The time constant of the fast component, which we identify as the readily releasable pool (RRP), was 19 ms (18 neuromasts, 10 zebrafish). Short, repeated pressure stimuli lead to progressive depletion of the RRP. Recovery from depletion showed a biphasic time course, with a fast time constant of 129 ms and a slow time constant of 2.8 s (33 neuromasts, 16 zebrafish). The relatively fast adaptation of synaptic responses (21 hair cells, 15 neuromasts, 7 zebrafish) was likely due to vesicle depletion, as the hair cell calcium responses (39 hair cells, 18 neuromasts, 4 zebrafish) did not show any significant decline throughout the stimulation. Both the calcium responses in the afferent terminals (37 afferent terminals, 17 neuromasts, 5 zebrafish) and the firing rate in the afferent ganglion neurons (32 neurons, 29 zebrafish) showed adapting responses comparable to fast adaptation at the hair cell ribbon synapse.   Conclusions We conclude that the hair cell synaptic release properties allow the lateral line to maintain its sensitivity in the presence of sustained stimuli, such as those produced by a constant flow of water.