We investigated synaptic plasticity in persons with Down' syndrome (DS) and control subjects used paired associative stimulation (PAS) protocol, a paradigm capable of producing long-term potentiation (LTP)-like changes in the sensorimotor system. After PAS, patients showed less LTP-like plasticity compared to control subjects. Abnormal motor cortex synaptic plasticity may play a role in the development of motor signs in DS.