Before the onset of hearing (postnatal day 12 in mice), inner hair cells (IHCs) are transiently innervated by medial olivocochlear (MOC) efferent fibers and fire spontaneous sensory-independent action potentials that are essential for the normal development of the auditory pathway. Several studies suggest that this spontaneous activity is driven and/or modulated by ATP released from cochlear supporting cells. Our work aims to investigate the role of ATP in modulating MOC-IHC synapses using pharmacology and electrophysiology. IHCs from BALB/C mice at P8-11 were patch-clamped, MOC fibers extracellularly stimulated, and its quantal content (m) estimated. Our results showed that ATP reversibly decreased m in a concentration-dependent manner (1 μM: 90±7 %, 10 μM: 60±7 %; 50 μM: 48±10 %, 100 μM: 48±6 %). Suramin, a non-specific P2 antagonist, abolished the effect of ATP. PPADS, an antagonist with a preferential effect on P2X receptors, and TNP-ATP, a specific P2X antagonist, did not modify ATP-induced inhibition. Furthermore, α,β-MeATP, a specific P2X agonist, had no effect on m. Both non-hydrolyzable ATP analog ATPγS, and the specific P2Y agonist 2-MeSADP, mimicked the effect of ATP (50 μM: 61±13 % and 10 μM: 72±7 % of control m, respectively). MRS2500, a specific P2Y1 antagonist, was used, but the ATP-induced inhibition persisted. Recent RNAseq studies indicate the expression of P2Y12 and 14 in MOC neurons, and thus specific antagonists of these subunits will be evaluated.