Somatostatin and neuropeptide Y concentrations have previously been reported to be increased in the basal ganglia in Huntington's disease (HD). In the present study we have extended these findings by examining both somatostatin-like immunoreactivity (SLI) and neuropeptide Y-like immunoreactivity (NPYLI) in cases of HD, which were graded according to the severity of pathological degeneration in the striatum. In addition, we surveyed a large number of subcortical nuclei and cortical regions for alterations. Both SLI and NPYLI were significantly increased about threefold in the caudate, putamen, and the nucleus accumbens. Increases in mild and severe grades were similar, which is consistent with a relative but not absolute sparing of striatal aspiny neurons in which somatostatin and neuropeptide Y are colocalized. Significant increases of NPYLI were also found in the external pallidum, subthalamic nucleus, substantia nigra compacta, claustrum, anterior and dorsomedial thalamus, bed nucleus of the stria terminalis, and locus ceruleus. SLI was significantly increased in the external pallidum, red nucleus, and locus ceruleus. Measurements of both neuropeptides were made in 24 regions of the cerebral cortex. Significant increases in both NPYLI and SLI were found in the frontal cortex (Brodmann areas 6, 8, 9, 10, 11, and 45) and temporal cortex (Brodmann area 21), whereas NPYLI was also increased in Brodmann areas 12, 20-22, 25, and 42. Alterations in the cerebral cortex were as pronounced in cases with mild striatal pathological changes as in those with severe striatal pathological changes. These alterations may occur early in HD and could reflect a selective sparing of somatostatin-neuropeptide Y cortical neurons combined with cortical atrophy.(ABSTRACT TRUNCATED AT 250 WORDS)