Amitotic [3H]thymidine-labeled C6 glioma cells, which are known to produce neurotrophic factor(s), were grafted alone and with adrenal chromaffin cells in an attempt to improve chromaffin cell survival and phenotypic differentiation. Long-Evans rats with unilateral 6-hydroxydopamine-induced lesions of the nigrostriatal pathway were divided into four groups: (1) those receiving adrenal medullary cells co-transplanted with C6 glioma cells; (2) those receiving adrenal medullary graft alone; (3) those receiving C6 glioma grafts alone; and (4) those serving as a vehicle control group. All rats were killed one month after transplantation. Immunohistochemical, neurochemical, and autoradiographic methods were used to identify and characterize the grafted cells. Tyrosine hydroxylase-immunoreactive cells were found in all animals that received grafts of the adrenal medulla alone or of adrenal medulla co-transplanted with C6 glioma cells. The cograft recipients had more tyrosine hydroxylase-immunoreactive cells than the hosts receiving just adrenal chromaffin cells (P less than 0.05). Additionally, more grafted chromaffin cells formed processes in the former group. All three tissue recipient groups (adrenal medullary, C6 glioma cell, and cografted animals) had a significant reduction (P less than 0.05) in ipsilateral rotations after amphetamine (0.5 mg/kg i.p.) injections as compared to the control vehicle recipient group. Moreover, the reduction in rotation was more marked in the cografted hosts than in the other two implanted groups (P less than 0.05). Significantly higher dopamine levels were found in the transplant sites of both cograft and adrenal medullary graft recipients than in sham grafted control animals.