Chronic L-DOPA treatment for Parkinson's disease often results in the development of abnormal involuntary movement, known as L-DOPA-induced dyskinesia (LIDs). Studies suggest that LIDs may be associated with aberrant cortico-striatal plasticity. Using in vivo extracellular recordings from identified type I and type II medium spiny striatal neurons, chronic L-DOPA treatment was found to produce abnormal cortico-striatal information processing. Specifically, following chronic L-DOPA treatment in dopamine-depleted rats, there was a transition from a cortically-evoked long-term depression (LTD) to a complementary but opposing form of plasticity, long-term potentiation, in type II `indirect' pathway neurons. In contrast, LTD could still be induced in type I neurons. Interestingly, the one parameter that correlated best with dyskinesias was the inability to de-depress established LTD in type I medium spiny striatal neurons. Taken as a whole, we propose that the induction of LIDs is due, at least in part, to an aberrant induction of plasticity within the type II indirect pathway neurons combined with an inability to de-depress established plastic responses in type I neurons. Such information is critical for understanding the cellular mechanisms underlying one of the major caveats to L-DOPA therapy.