The term dementia refers to a number of very complicated disorders of the brain with several morphological, biochemical and functional alterations. Several alterations occur in Alzheimer's disease. They include alteration in neurotransmitters related substances and changes in brain metabolic rate. Changes in metabolic activity are among the earliest and best documented of all modification reported in Alzheimer's brain. They appear to be correlate with the severity of the disease, and to be more marked in specific areas of the brain which appear to be clinically more involved. In this report will be discussed the observations on changes of cerebral glucose metabolic rate, cerebral oxygen consumption, cerebral blood flow reported in vivo in demented patients. The observations of changes in energy metabolism related enzymes and the relationship between these changes and those in neurotransmitters related substances will be also reviewed.