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Publisher Summary This chapter presents a study examining the origin and projections of a spinal nociceptive and thermoreceptive pathway. In this study, injections of 0.2 to 3.0 μl of 30 or 50% horseradish peroxidase (HRP) in sterile water or saline were made unilaterally under aseptic conditions in animals anesthetized with sodium pentobarbital. The HRP was delivered by means of 1 to 3 penetrations of a 1 or a 10 μl Hamilton microsyringe needle that was placed stereotaxically through a trephine hole in the calvarium. In the later experiments, the final placement of the microsyringe needle was determined electrophysiologically by searching for evoked potentials with a recording electrode while stimulating a contralateral forelimb and hindlimb cutaneous nerve percutaneously. The findings illustrate the extent of spread of HRP injected into the brainstem of two monkeys. In M-6, most of the nuclei in the posterior and lateral thalamus were involved because of the amount of HRP injected. Because most of the thalamic nuclei that receive terminals of the spinothalamic tract were involved, a large number of retrogradely labeled neurons were found throughout the spinal cord of this monkey. In M-12, the HRP remained restricted primarily to the periaqueductal gray and adjacent reticular formation.