Responses of striatal neurons to stimulation in substantia nigra were recorded intracellularly in intact rats and after acute or chronic unilateral lesions of cerebral cortex or after combined cortical lesions and unilateral thalamic transections. Spiny striatal efferent neurons were identified by intracellular injection of horseradish peroxidase. In intact animals substantia nigra stimulation evoked a complex response with both excitatory and inhibitory phases. Acute unilateral decortication abolished the inhibitory phase of the response and reduced the amplitude of the initial EPSP. Thus, part of the excitatory phase and most or all of the inhibitory phase of the response result from polysynaptic routes to striatum involving cerebral cortex. The remaining EPSP observed in acute decorticate animals exhibited two components distinguished on the basis of their time courses. The latter of these was abolished by thalamic transections. The earlier component was shown to be a monosynaptic EPSP evoked by axon collaterals of cortical efferent neurons projecting to brainstem and was not observed in animals subjected to chronic decortication. After removal of all of these non-nigral response components a small long latency EPSP could be evoked by nigral stimulation. The EPSP is probably due to activation of dopaminergic nigro-striatal axons.