Synapse elimination occurs throughout the nervous system during development, and is essential for the formation of neural circuits. The mechanisms underlying synapse elimination in the brain, however, remain largely unknown. Using whole-cell patch-clamp recording in a slice preparation, we examined synaptic refinement at the somatosensory relay synapse (lemniscal synapse) in the ventral basal thalamus of the mouse during postnatal development. At one week old, each neuron in the ventral basal thalamus is innervated by multiple lemniscal fibers, as revealed by multiple increments of the synaptic response. By 16 days after birth (P16), the majority of neurons showed an all-or-none response, suggesting a single fiber innervation. In addition to synapse elimination, extensive modifications in synaptic properties occur during the second week after birth. The ratio of AMPA to NMDA component of the synaptic current tripled between P7 and P17. The decay constant of the NMDA component decreased by about 70% between P7 and P17; ifenprodil (3 microM) reduced NMDA component by about 40% in neurons at P7-9, but was much less effective at P20-24. On the other hand, there was little change in the inward rectification of AMPA component between P11 and P24. Paired-pulse ratios, measured at -70 and +40 mV, were stable between P7 and P24. Whisker-deprivation from P5 through P19 had no effect on the elimination or the maturation of the lemniscal synapse. These results suggest that the lemniscal synapse in the ventral basal thalamus undergoes extensive refinement during the second week, and that sensory experience has a rather limited role in this process.