We examined the structural plasticity of excitatory synapses from corticostriatal and thalamostriatal pathways and their postsynaptic targets in adult Sprague-Dawley rats to understand how these striatal circuits change in l-DOPA-induced dyskinesias (LIDs). We present here detailed electron and light microscopic analyses that provide new insight into the nature of the structural and synaptic remodeling of medium spiny neurons in response to LIDs. Numerous studies have implicated enhanced glutamate signaling and persistent long-term potentiation as central to the behavioral sensitization phenomenon of LIDs. Moreover, experience-dependent alterations in behavior are thought to involve structural modifications, specifically alterations in patterns of synaptic connectivity. Thus, we hypothesized that in the striatum of rats with LIDs, one of two major glutamatergic pathways would form new or altered contacts, especially onto the spines of medium spiny neuron (MSNs). Our data provide compelling evidence for a dramatic rewiring of the striatum of dyskinetic rats and that this rewiring involves corticostriatal but not thalamostriatal contacts onto MSNs. There is a dramatic increase in corticostriatal contacts onto spines and dendrites that appear to be directly linked to dyskinetic behaviors, since they were not seen in the striatum of animals that did not develop dyskinesia. There is also an aberrant increase in spines receiving more than one excitatory contact(i.e., multisynaptic spines) in the dyskinetic animals compared with the 6-hydroxydopamine-treated and control rats. Such alterations could substantially impair the ability of striatal neurons to gate cortically driven signals and contribute to the loss of bidirectional synaptic plasticity.