L-[4,5-3H]leucine was injected stereotaxically into various regions of the rat neostriatum. Light microscopic autoradiographic techniques were used to plot the entire efferent pathways of the neostriatum, and in particular, the projections to the substantia nigra (SN). The terminal distribution of the pathways projecting to the ipsilateral SN was predominantly restricted to the zona reticulata region. The dorsal part of the head of the striatum was found to innervate the anterior and medial portions of the zona reticulata, while the ventral area of the head projected to more posterior regions of the SN. A pathway from the tail of the striatum to the lateral and dorsal parts of SN was also demonstrated. Horseradish peroxidase injections, restricted to different areas of the SN, led to retrograde labelling of neurons in the striatum whose distribution confirmed the topographic organization of the striatonigral pathway demonstrated in the autoradiographic studies.