Neurons in the superficial dorsal horn of the spinal cord are important for conveying sensory information from the periphery to the central nervous system. Some synapses between primary afferent fibres and spinal dorsal horn neurons may be inefficient or silent. Ineffective sensory transmission could result from a small postsynaptic current that fails to depolarize the cell to threshold for an action potential or from a cell with a normal postsynaptic current but an increased threshold for action potentials. Here we show that some cells in the superficial dorsal horn of the lumbar spinal cord have silent synapses: they do not respond unless the holding potential is moved from -70 mV to +40 mV. Serotonin (5-hydroxytryptamine, 5-HT), an important neurotransmitter of the raphe-spinal projecting pathway, transforms silent glutamatergic synapses into functional ones. Therefore, transformation of silent glutamatergic synapses may serve as a cellular mechanism for central plasticity in the spinal cord.