Dysfunction of the nigrostriatal dopamine system results in marked disorders of movement such as occur in Parkinson’s disease. Functions of this dopamine-containing projection system were examined in monkeys trained in a classical conditioning task, and the effects of striatal dopamine depletion were tested. Unilateral dopamine loss substantially reduced the acquired sensory responsiveness of striatal neurons monitored electrophysiologically. This effect was ipsilateral and selective, and could be reversed by apomorphine. These results suggest that the primate nigrostriatal system modulates expression of neuronal response plasticity in the striatum during sensorimotor learning.