This is a prospective, multicenter, open-label study. Following screening and baseline assessments, eligible patients will be switched to rivastigmine and will enter the 16 week run-in rivastigmine treatment phase. After completion of assessments at the end of the run-in phase, patients who were not sufficiently stabilized on rivastigmine alone will receive add-on memantine to their rivastigmine treatment; patients who were stabilized on rivastigmine alone will have completed and be discontinued from the study.

Inclusion Criteria:

• Outpatients who have probable Alzheimer's disease according to the DSMIV criteria
• Patients treated with donepezil (5-10 mg ) or galantamine (16- 24 mg) for at least 6 months
• Patients, in the investigator's clinical judgment, not stabilized on treatment with donepezil or galantamine

Exclusion Criteria:

• Patients with evidence of severe or unstable physical illness, i.e., acute and severe asthmatic conditions, severe or unstable cardiovascular disorders, active peptic ulcer disease, hypersensitivity to cholinesterase inhibitors or memantine, clinically significant laboratory abnormalities or any patient with a medical condition which would prohibit them from completing the clinical trial