Reductions in 2 neurotransmitter synthesizing enzymes in brain, glutamic acid decarboxylase (GAD) and choline acetyltransferase (CAT), have been found in dementias of different origins, including senile dementia (Alzheimer type). Significant reductions in cerebral GAD have also been found in depression (unipolar). The GAD reductions did not generally appear to be localised in any specific region of the brain examined. However, the reduction of CAT in the hippocampus, relative to reductions in other areas examined, was substantially greater in the brains with Alzheimer-type changes. GAD and CAT activities in normal brains were examined for the effects of some variable factors inherent in necropsy biochemical measurements. These factors included: (i) age; (ii) agonal status; (iii) time of death, and (iv) delay in tissue sampling; and GAD was found to be significantly influenced by (ii), (iii) and (iv) and CAT by (i), (iii) and (iv). None of these factors accounted for the total alterations in the enzyme activities of the mentally abnormal brains. The results indicate that reductions in cerebral GAD require to be interpreted with caution in view of the sensitivity of this enzyme to premortem status but that reductions in cerebral CAT may be a more reliable index of pathological change in senile (Alzheimer-type) dementia.