Autografts of adrenal medulla were implanted into preformed cavities in the caudate nuclei of four rhesus monkeys with hemiparkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Five other hemiparkinsonian monkeys underwent caudate cavitation, but received no tissue implant. All of the animals had marked bradykinesia of the affected arm and stable apomorphine-induced turning before cavitation or implantation. Moderate behavioral recovery was seen in all five monkeys with cavitation and two of the three monkey with long-term adrenal autografts (the fourth adrenal recipient was sacrificed 10 days after grafting). The improvement occurred months after the procedure and was not as early or as complete as that seen after fetal dopaminergic grafts. Surviving adrenal tissue was found only in the animal that showed no behavioral recovery. The other two adrenal autograft recipients (with no surviving adrenal medulla) and all of the animals with cavitation had ingrowth of dopaminergic fibers from the area olfactoria and nucleus accumbens into the caudate, oriented toward the cavity. These findings show that the mechanism of improvement after adrenal medullary implants for parkinsonism is not dopamine secretion by chromaffin cells, but may be related to the sprouted host fibers. The results also indicate that the limited recovery after adrenal implants in parkinsonian patients may be a result of the cavitation, and not necessarily the result of tissue implantation.