The present paper examines the relationship between choline acetyltransferase (ChAT) activity and gamma aminobutyric acid (GABA) concentrations with neuronal counts, senile plaque counts and estimates of neurofibrillary tangles in a series of 25 cases of senile dementia of Alzheimer type (SDAT) with appropriate controls. ChAT activity was significantly positively correlated with neuronal counts in the frontal and temporal region for the whole group but not in the demented or control subgroups. Significant negative correlations were also found between plaque counts and ChAT activity in all areas studied in the total group of subjects and in most of the SDAT groups. Significant negative correlations between ChAT activity and estimates of neurofibrillary change occurred in the frontal area and midtemporal gyrus in the SDAT cases. All the above correlations were seen most prominently in the youngest dementia patients (less than 79 years of age) who exhibited the most severe neuropathological and neurochemical deficits. The neurochemical distinction between the young and elderly age group of SDAT cases was well illustrated by the paradoxical trends of significantly increased GABA and ChAT levels with increasing age. The few significant correlations observed between GABA and cell counts were positive and occurred in the temporal lobe in the younger age group. Plaque counts and neurofibrillary tangle estimates showed little correlation with GABA concentrations.