The content of somatostatin-like immunoreactivity (SRIF-LI) and neuropeptide Y-like immunoreactivity (NPY-LI) has been measured in both control post-mortem human brains and in Huntington's disease brains. The content of both SRIF-LI and NPY-LI was found to be significantly increased in the basal ganglia of Huntington's disease brains compared with a control group. The nature of the SRIF-LI and NPY-LI in both control and Huntington's disease brains was investigated after separation on Sephadex G25 and G50 columns. Using a C-terminal-directed SRIF radioimmunoassay (RIA), 3 peaks of immunoreactivity were measured, whilst an N-terminal-directed SRIF RIA detected two peaks of immunoreactivity. In each case, the elution profile did not differ between control and Huntington's disease caudate nucleus. The content of immunoreactivity in each peak was found to be increased in Huntington's disease brains compared with controls. Only one peak of NPY-LI was detected in both control and Huntington's disease caudate after separation on Sephadex G25 and G50 columns. Immunohistochemical staining of the caudate and putamen of control and Huntington's disease brains revealed a population of neurones containing NPY-LI. The number of NPY-positive neurones was found to be increased in both the caudate and putamen of Huntington's disease brains compared to control caudate and putamen.