Choline acetyltransferase (ChAT), acetylcholinesterase (AChE) and glutamic acid decarboxylase (GAD) activities were measured in 20 brain regions from autopsied control and senile dementia of the Alzheimer type (SDAT) cases. Large, widespread reductions in the activities of ChAT and AChE were found in tissues from SDAT cases, while GAD activities were reduced in 3 of the 20 regions investigated. AChE activity in cerebrospinal fluid from SDAT cases was similar to that found in samples from non-demented patients.