This study will test the use of venlafaxine to treat the depression in Alzheimer's Disease. Venlafaxine works by increasing natural substances in the brain (serotonin and norepinephrine) that help maintain mental balance. Alzheimer's disease (AD) is the commonest neurodegenerative disease of aging and the cause of major financial and emotional burden to patients, families and caregivers, and society. Depression is a very common symptom of AD, affecting as many as 50% of patients over their illness. Depression in AD (Alzheimer's disease) contributes greatly to patient disability and caregiver distress. Neither psychosocial interventions nor psychotropic medications have proven effective to date for the treatment of depression in AD.Venlafaxine is approved by the U.S. Food and Drug Administration (FDA) for the treatment of major depression but it is not known whether or not it can help depression in Alzheimer's Disease.

Inclusion Criteria:

• Dementia due to Alzheimer's disease by Diagnostic and Statistical Manual Diploma in Social Medicine (DSM)-IV (TR) criteria (90), with a Mini-Mental State Exam (MMSE) (82) score of 10-26 inclusive;
• Depression as defined by the National Institute of Mental Health (NIMH) Consensus Criteria,
• Clinical Dementia Rating Scale of 1 "mild" or 2 "moderate". Ratings of 3 "severe" will be excluded because many of the instruments lack validity in the presence of severe cognitive impairment, particularly language deficits.
• Sufficiently good health to be treated using the study protocol in usual care circumstances;
• Patient or surrogate and caregiver provides informed consent for participation in the study;
• A caregiver is available who spends at least 10 hours per week with the patient, supervises her care, and is willing to accompany the patient to study visits and to provide information about the patient.
• Female participants must be at least 2 years post menopause or surgically sterilized. Exclusion Criteria
• Presence of a brain disease that might otherwise fully explain the presence of dementia, such as stroke, Parkinson's disease, traumatic brain injury, multiple sclerosis, and similar neurologic diseases;
• Clinically significant psychosis that requires antipsychotic treatment; -Treatment with venlafaxine is contraindicated in the opinion of the attending psychiatrist, for example if there is a prior history of dangerous or -unacceptable side effects when treated with venlafaxine;
• Failure of treatment with venlafaxine in the past for depression after convincing evidence of a "good trial," for example 8 weeks at the highest tolerated dose;
• Treatment for a condition or with a medication that would prohibit the safe concurrent use of venlafaxine (specifically including systolic blood pressure > 180 mm Hg or diastolic blood pressure > 100 mm Hg);
• Diagnosis of congenital long Q-T syndrome
• The patient requires psychiatric hospitalization for depression or is suicidal;
• Initiation, discontinuation or dose changes in cholinesterase inhibitor or memantine use within the 4 weeks prior to screening.