Human fetal mesencephalic tissue was grafted to rats with unilateral lesions of the nigrostriatal pathway. The animals were immunosuppressed with cyclosporine A. Grafts were placed either into the lateral ventricle ipsilateral to the lesion or in the cingulate cortex above corpus callosum. The grafts and newly formed fibers were visualized by immunohistochemistry with antibodies against tyrosine hydroxylase (TH) and the human Thy-1 glycoprotein. TH-positive fibers covered the total volume of striatum when the graft was placed either in the ventricle or in the cortex. When the transplant was located in the ventricle, TH-positive cells migrated from the graft into host striatum. No cell migration was seen into any other areas than striatum. Cortex and septum were sparsely reinnervated by the graft, but not to a density higher than that normally seen. Globus pallidus was totally devoid of TH-positive fibers. When the graft was placed in cingulate cortex, fiber bundles penetrated through corpus callosum into either striatum, to arborize in its dorsal parts, or followed the medial side of the lateral ventricle to ventral limbic areas, where a fiber network also was formed. Human specific Thy-1-immunohistochemistry revealed positivity only on the lesioned side. These data suggest that dopamine neurons in human mesencephalic tissue, grafted to the rat brain, can migrate specifically into host striatum. Furthermore, TH-positive fiber outgrowth occurred only into dopamine denervated areas of the host, avoiding areas that are normally not innervated by nigral neurons, but also able to reach distant target cells.