Effect of transplantation of embryonic ventral mesencephalon preparation containing dopaminergic neurons on repair of the dopaminergic nigrostriatal system was studied in rats with hemiparkinsonism induced by 6-hydroxydopamine. Transplantation of embryonic ventral mesencephalon into denervated striatum led to a more than 50% decrease in apomorphine-induced rotation, recovery of dopamine and DOPAC levels in the brain, and to an increase in DOPAC excretion and the DOPAC-dopamine ratio in daily urine of rats with hemiparkinsonism. Dopaminergic neurons of the transplant survived, forming a network of tyrosine hydroxylase-positive processes growing beyond the transplant and reinnervating the adjacent compartments of the striatum. A positive correlation between urinary excretion of DOPAC and brain concentration of dopamine was revealed in denervated rats after transplantation of ventral mesencephalon. Intrastriatal transplantation of cell preparations of embryonic striatum containing no dopaminergic neurons and isolated local injury to the striatum did not affect regeneration of the denervated nigrostratal system.