Levodopa-induced dyskinesias are clinically heterogeneous, both in appearance and timing with respect to dose. Electromyogram observations indicate that levodopa-induced dyskinesias are comprised of irregular bursts of either synchronous or asynchronous neuronal firing in antagonist muscles. Studies of the blink reflex and spontaneous blinking have provided useful neurophysiologic information on brainstem function that is sensitive to changes in brain dopamine concentrations. The blink rate is reduced in Parkinson's disease (PD) and increased with dopamine treatment. The blink rate in patients with levodopa-induced dyskinesias, however, has been shown to be faster than that in optimally treated PD patients and normal individuals. These results suggest that dyskinesias are associated with a relative hyperdopaminergic state. However, there appears to be no correlation of dopaminergic benefit to the parkinsonian symptoms, indicating perhaps that there are several dopaminergic systems, including one responsible for motor function and one for dyskinesia. Alternatively, it may be that the pattern of neural firing influences dyskinesias, while the average firing rate may be responsible for motor benefits.