This chapter discusses certain histochemical properties and behavioral effects of unilateral and the bilateral transplantations of fetal dopamine (DA)-containing neurons to the weaver neostriatum. The delayed cavity transplantation protocol is used throughout the described experiments in the chapter; donor tissue had been obtained from the ventral mesencephalic flexure of embryonic day (E) 14-El5 wild-type fetuses of the same inbred mouse strain as the recipient mutants. Grafts of fetal mesencephalon survive and express tyrosine hydroxylase-immunoreactive (TH-ir) following transplantation to the weaver striatum. Surviving grafts contain on the average 200–1200 TH-ir neurons. Grafted cells are located in the cortical aspiration cavity above the striatal complex of the host. The expression of TH, the rate-limiting enzyme in the biosynthesis of DA, is indicative of the presence of DA in transplanted neurons. Following the transplantation of solid mesencephalic grafts or mesencephalic cell suspensions to the weaver striatum, TH-ir axon terminals have been found in contact with post-synaptic elements of the host striatum.