RESUMEN
The marker horseradish peroxidase (HRP) was used to retrogradely label the cell bodies of origin of the nerve fibers responsible for thymic innervation. The thymus was exposed in albino rats under general anesthesia and a conjugate of HRP-WGA was injected bilaterally into the gland parenchyma. After a survival time of 3 days, the animals were anesthetized and perfused transcardially. Their brainstems were removed and serially sectioned and together with the whole sympathetic chains were processed for HRP demonstration. Labeled cell bodies were identified in the brainstem at the level of the retrofacial and ambiguous nuclei bilaterally. In the sympathetic chain, labeled neurons were present from the first cervical to the ninth thoracic ganglion, although they were detected in higher numbers in the superior cervical and stelate ganglia.