In the present work fetal conditionally immortalized striatal cells ST14A and human mesencephalic neural progenitor cells (NPCs) have been studied with respect to proliferation and differentiation in in vitro experiments. Furthermore, in in vivo experiments, human NPCs have been investigated after transplantation into neonatal and adult hemiparkinsonian rat striatum. In the transplantation studies the main focus laid on the survival, migration and dopaminergic (DAergic) differentiation of human NPCs in the host brain. The cells behaved differently when transplanted into neonatal or adult hemiparkinsonian rat striatum, demonstrating better survival and migration over longer distances in the neonatal rat striatum. Moreover, some cells assumed DAergic phenotype in neonatally transplanted rats, whereas the same cells showed no DAergic differentiation when transplanted into the adult striatum. Nevertheless, improvement in lesion-induced motor impairment has been demonstrated in both groups of hemiparkinsonian rats, independently from DA-ergic differentiation, indicating, that other factors expressed by grafted cells may also play a role in recovery.