Previous studies have shown that in advanced cases of Huntington's disease, enkephalin-immunoreactive striatal projections to the external globus pallidus may be more affected than substance P-containing striatal projections to the inner segment of the pallidum [Reiner A. et al. (1988) Proc. natn. Acad. Sci. U.S.A., 85, 5733–5737]. Other immunohistochemical [Ferrante R. J. et al. (1990) Soc. Neurosci. Abstr., 16, 1120] and neurochemical observations [Storey E. and Beal M. F. (1993) Brain, 116, 1201–1222] suggest no difference in the loss of these peptide-containing pathways in Huntington's disease. In view of the potential significance of this issue for understanding the neuropathological process in Huntington's diseasem we examined the globus pallidus in control and Huntington's disease brains, using a quantitative approach which involved high resolution image analysis of 7 μm frozen sections to determine the overall density of peptide-immunoreactive terminals. Results showed that in the controls there was no significant difference between the density of enkephalin- and substance P-immunoreactive terminals in the external and internal globus pallidus, respectively. In all Huntington's disease brains, including grade 1 cases, enkephalin-immunoreactive terminals in the external globus pallidus were significantly reduced compared to substance P-positive boutons in the internal segment of the adjacent section. In comparison to controls, enkephalin immunoreactivity in all Huntington's disease cases was significantly lower; substance P-immunoreactive terminals in the internal globus pallidus were significantly lower than controls in some of the grade 2 cases and in the grade 3 cases. Our results using a method of analysis different from previous studies support the earlier findings of Reiner et al., that in Huntington's disease the striatopallidal pathway containing immunoreactive enkephalin is affected to a greater extent than the projection containing substance P. Moreover, the additional finding in this study that enekephalin and substance P projections differ markedly in the grade 1 cases strongly suggests that the alterations in the enkephalin pathway signal a relatively early event in the neuropathological process in Huntington's disease.