The extracellular single unit responses of striatal neurons to repetitive stimulation of the sciatic nerve were recorded in the urethane anesthetized rat. Changes in the magnitude of these responses after pharmacological manipulation of dopamine (DA) neurotransmission were evaluated. While the intravenous administration of 0.25 mg/kg i.v. amphetamine (AMPH) had no significant effect on baseline firing rates as compared to saline controls, the magnitudes of excitatory and inhibitory evoked single unit responses were significantly decreased by 68% by AMPH. Further, this reduction in response magnitude produced by AMPH was completely blocked by pretreatment with 0.5 mg/kg i.v. haloperidol or by intrastriatal 6-hydroxydopamine lesions. This indicates that the observed effect is mediated by dopamine located in nerve terminals within the striatum. These results suggest that DA functions to modulate the responsiveness of striatal neurons to afferent signals.