Fourty cases of patients with dementia, and 15 cases of normaly properly matched control group. were submitted for estimation of platelet M.A.O activity. the patient classified in 24 cases withS.D.A.T and 16 cases M.I.D by a special cognitive capacity screening test(C.C.S.T) and the modified Hachinski's Ischaemic index and the diagnosis was confirmed by C.T scan. the mean platelet M.A.O activity was significantly higher in the demented compared to the control group on the same time this showed a highly significant increase in the S.D.A.T group compared to the control and the M.I.D group did not show a significant statistical increase.