Acetyl- and butyryl-cholinesterase activities have been measured biochemically in normal brain tissue, in senile dementia of Alzheimer type and in mental disorders without Alzheimer-type abnormalities. Acetylcholinesterase was significantly reduced and butyrylcholinesterase significantly increased, compared with the normal, in the hippocampus and temporal cortex of the Alzheimer cases. No significant enzyme changes were seen in the other diseases investigated including multi-infarct dementia, schizophrenia and depression. There was no correlation between age and acetylcholinesterase activity, but a significant positive correlation between the butyrylcholinesterase activities with increasing age (60-90 years) was found in the hippocampus. The possible connection between cholinergic system pathology and these cholinesterase abnormalities in Alzheimer dementia is discussed.