Because synaptic refinement of medial nucleus of trapezoid body (MNTB) - lateral superior olive (LSO) synapses is most active during the first postnatal week and the long term depression (LTD) has been suggested as one of its mechanisms, LTD of MNTB-LSO synapses was investigated in neonatal rat brain stem slices with the whole cell voltage clamp technique. In Mg2⁢ free condition, tetanus (10 stimuli at 10 Hz for 2 min) in the current clamp mode induced a robust LTD of isolated D, L-APV-sensitive postsynaptic currents (PSCs) for more than 30 min (n=6, 2.4⁑0.4% of the control), while isolated CNQX-sensitive PSCs were not suppressed (n=6, 95.3⁑1.6%). Tetanus also elicited similar LTD in the isolated GABAergic/glycinergic PSCs (n=5, 3.6⁑0.5%) and mixed PSCs (GABAergic/glycinergic/glutamatergic) (n=4, 2.2⁑0.7%). However, such a strong LTD was not observed in the mixed PSCs when 10 mM EGTA was added in the internal solution (n=10), indicating that postsynaptic Ca2⁢ rise is needed for the strong LTD. This robust LTD might contribute to the active synaptic refinement occurring during the first postnatal week.