Electrical stimulation of the substantia nigra of rats elicits a burst of small amplitude waves with a latency of 4-6 ms that may last for 10-15 ms throughout much of the neostriatum. Frontal cortex stimulation also elicits a burst response, which can occlude the substantia nigra response. The substantia nigra evoked burst response was still present after chronic neocortical ablation or thalamic transection or both treatments combined. The response corresponds to the first sharp negative wave of the substantia nigra evoked neostriatal field potential. Single substantia nigra evoked action potentials were recorded in neostriatum with a mean latency of 9.8 ms, ranging from 4-22 ms. These action potentials were considered to be antidromic because they were occluded during appropriate collision intervals by orthodromic action potentials elicited by frontal cortex stimulation. Subthreshold frontal cortex conditioning stimulation did not alter the threshold for activation from substantia nigra. The refractory period for the axon was at least as long as that for the soma and ranged between 0.8-2.0 ms. The antidromic responses failed to follow low frequency stimulation (less than 40 Hz for 3000 ms). This failure occurred in the axon between substantia nigra and globus pallidus. The burst response and first sharp negative wave of the field potential probably represent the antidromic activation of the ubiquitous and densely packed medium spiny neostriatal projection neurons. These responses occur at the same latency, respond in the same manner to twin pulse and repetitive stimulation and are occluded by frontal cortex stimulation in the same manner as antidromic action potentials.