Cell suspensions of embryonic brainstem containing the locus coeruleus were injected intervertebrally into the lumbar spinal cord of adult rats whose descending catecholamine (CA) fibers had been lesioned with intracisternal injection of 6-hydroxydopamine. Up to 1100 CA cells were found 2 and 4 months later, and these cells grew processes which produced histologically detectable reinnervation of the lumbar gray matter on the injected side of the cord. To assess the functional activity of the transplanted CA cells, the force of the hindlimb flexion reflex was measured in acute spinal rats. This reflex has been shown previously to be strongly enhanced by catecholamines. The flexion reflexes were significantly stronger in the transplanted rats than in the controls. Further, the flexion reflexes were significantly reduced by phenoxybenzamine, an alpha-adrenergic blocker, in the transplanted rats while the reflexes of controls were not significantly changed. These results demonstrate that cell suspension transplants of embryonic brainstem containing the locus coeruleus into the adult rat spinal cord survive, grow reinnervating catecholamine processes, and can affect the functional activity of the spinal cord.