Objective — to analyze the activity of acetylcholinesterase (AChE), butyrylcholinesterase (BChE) and neprilysin (NEP) in the blood serum in elderly people with different types of cognitive impairment and evaluate the effect of ceraxon on the biochemical parameters. Material and methods. Three groups of patients: without cognitive disorders (controls — CG), with amnestic mild cognitive impairment (а-MCI) and with Alzheimer’s disease (AD were studied). Results and conclusion. The activity of AChE, BChE and NEP was reduced in the blood serum of patients with a-MCI and, to the greater extent, in patients with AD compared to CG and correlated with the level of cognitive dysfunction evaluated by MMSE, ADAS-cog, and other tests. For the first time, it has been shown that treatment of a-MCI patients with ceraxon (citicolin) results in an increase of the activity of blood serum AChE, BChE and NEP to the values observed in the CG. Thus, the activities of blood serum AChE, BChE and NEP reflect the level of cognitive dysfunction and can be used as prognostic biomarkers of the level of dementia progression in patients with impaired memory.