Noradrenaline, dopamine, homovanillic acid (HVA), choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) levels were measured in post-mortem brains from 8 cases of pre-senile Alzheimer-type dementia (ATD), 5 cases of senile ATD, 4 cases of Down's syndrome aged 53-57 years, one 27-year-old case of Down's syndrome and 13 controls. In the controls, the concentration of noradrenaline in hypothalamus (P less than 0.05) and mamillary body (P less than 0.02) decreased with age. Compared with age-matched controls, noradrenaline levels in these areas were more markedly reduced in pre-senile ATD (P less than 0.01), the 53-57-year-old cases of Down's syndrome (P less than 0.001) and in the 27-year-old Down's, than in senile ATD (hypothalamus P less than 0.05, mamillary body, n.s.). Dopamine and HVA concentrations in caudate nucleus were unaltered in pre-senile or senile ATD but dopamine was decreased (P less than 0.01) in the older cases, although not in the 27-year-old case, of Down's syndrome. In the olfactory tubercle in ATD the level of HVA was unaltered but the activity of ChAT was decreased (P less than 0.01). ChAT activity was reduced in pre-senile ATD (P less than 0.001), the older Down's cases (P less than 0.01) but not the young Down's case, and senile ATD (P less than 0.001) in the temporal cortex and in pre-senile ATD (P less than 0.001) and the older Down's cases (P less than 0.001) but not senile ATD in the caudate nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)