Short Term Effects of PRX-03140 in Patients With Mild Alzheimer's Disease Being Treated With Aricept

Inclusion Criteria: * At least 60 years old. * On a stable 10 mg dose of Aricept for at least 90 days and able to take it in the morning. * Diagnosis of probable Alzheimer's Disease. * Mild dementia. * You or your authorized representative provide voluntary written informed consent. * Not pregnant, planning a pregnancy, or capable of becoming pregnant. Exclusion Criteria: * Any other medical condition which, in the opinion of the Investigator, would jeopardize your safety, impact the validity of the study results, or interfere with your ability to complete the study according to the protocol. * Intolerance to Aricept. * Dementia other than Alzheimer's type. * Parkinson's Disease. * History of seizure or epilepsy. * History of stroke. * Participation in another research study within last 30 days. * Enrollment in any previous research study testing PRX-03140. * Use of prescription cognitive enhancing medications (except Aricept) or MAO inhibitors within last 4 weeks. * Use of tobacco products within last 4 weeks. * Positive blood screen for Hepatitis B surface antigen or Hepatitis C antibody. * Positive urine screen for alcohol or drugs of abuse or history of drug or alcohol abuse within last 6 months. * Major surgery within last 4 weeks.