Severe cases of neonatal jaundice are known to cause complications like epileptic seizures and deafness. But the underlying mechanisms for this hearing loss have not been elucidated. Here we use the Gunn rat model of hyperbilirubinaemia to investigate the detrimental effect of high levels of bilirubin on the central auditory pathway, focusing on synaptic transmission in the Medial Nucleus of the Trapezoid Body (MNTB). Auditory brainstem responses (ABR) revealed a significant loss of sound-evoked brainstem activity 18hrs after jj-Gunn rats were treated with sulfadimethoxine (200mg/kg) which caused elevated bilirubin levels (1). All the procedures were performed in accordance with UK Animals (Scientific Procedures) Act 1986. For in vivo ABR measurements rats were anaesthetised with a combination of Hypnorm (VetaPharma Ltd, fentanyl (0.15mg/kg)+ fluanisone (5mg/kg)) and Hypnovel (Roche, 2.5mg/kg) i.p. In vitro extracellular field potential recordings (as described previously (2)) from the MNTB of treated animals showed an increased delay in synaptic transmission, while whole-cell patch-clamp recordings from MNTB principal neurons confirmed that treated neurons were healthy and capable of firing normal action potentials. However synaptic stimulation failed to elicit large EPSCs from calyceal inputs (1.5±0.48nA, n=9, mean±SEM) in jj-Gunn rats exposed to elevated bilirubin levels;in contrast, MNTB neurons from control untreated jj-Gunn rats showed characteristic large calyceal EPSCs (9.6±1.21nA, n=9, mean±SEM, at -60 mV) following synaptic stimulation. Anterograde labelling of the calices with dextran-rhodamine revealed morphological changes indicating severe degeneration in the presynaptic terminals from high-bilirubin treated Gunn rats, resulting in a reduction of surface area (control: 2717.88μm2±204.6, n=20 vs. treated: 1018.5μm2±171.8, n=13, mean±SEM, p>0.001, Mann-Whitney-U-test). Electron microscopic images confirmed the morphological changes observed in living tissue. These observations support the notion that deafness observed during neonatal jaundice is due to the failure of excitatory synaptic transmission at the presynaptic rather than the postsynaptic site.