Transplantation of embryonic substantia nigra into the adult rat brain decreases the motor asymmetry that is produced by dopamine receptor supersensitivity after a unilateral lesion of the substantia nigra. The authors report that this effect of transplantation is specific to grafts of substantia nigra. They also report that, in conjunction with the decrease in motor asymmetry, these grafts cause postsynaptic dopaminergic binding sites to return to normal density as measured by tritiated spiroperidol autoradiography. Thus, in animals with brain lesions, grafts of substantia nigra produce a long-term alteration in the functional status of host brain cell receptors that is associated with a reduction in the behavioral deficit.