Motor deficits produced in rats by unilateral substantia nigra lesions have been found to be reduced by grafts of fetal rat substantia nigra to the dopamine-denervated caudate nucleus. In the present study these grafts were examined behaviorally, histochemically, and biochemically over six- to 10-month periods. The grafts were found to survive in a healthy condition and contain catecholaminergic cells and fibers after eight to ten months. Concentrations of dopamine in adjoining parts of the caudate nucleus were increased when examined six months after grafting. Apomorphine-induced rotation was reduced by the grafts, and these reductions persisted for at least six months. Although signs of aging were observed in the brains of the host animals when sacrificed eight to ten months after grafting, the grafts remained healthy and showed no signs of aging or deterioration. It is concluded that substantia nigra grafts can become permanent, functional constituents of the brains of host animals with prior substantia nigra lesions.