• This is an exploratory study. The primary efficacy point will be global impressions. [ Time Frame: baseline versus study end ] [ Designated as safety issue: Yes ]
  

• Analyses will be computed for the categorical dependent variables (DV): spirograph, pouring, subjective ADL, observed tremor, global assessment by examiner, global assessment by patient, and subjective assessment by patient scores. [ Time Frame: baseline versus end of study ] [ Designated as safety issue: Yes ]
  

Inclusion Criteria:

1. Subjects must be between the ages of 18 and 80 inclusive.
2. Each subject must meet standard criteria for PD.
3. All patients on dopaminergic therapy must report benefit. -No other abnormal neurological signs. -No direct or indirect trauma to the nervous system within 3 months preceding the onset of PD. -No convincing evidence of sudden onset or evidence of stepwise deterioration.
4. Subjects must be in generally good health as evidenced by previous medical history and clinical examination.
5. Subjects will be allowed to take any PD medication with the exception of amantadine. They will also be allowed to take medications approved for the use of Alzheimer's disease.
6. Subjects will be required to be on a stable dose of all medications for at least two weeks prior to entry into the study and may not alter these medications throughout the study.
7. If subjects are on an anti-depressant medications, a stable dose of these will be required for at least six weeks prior to entry into the study.
8. Subjects must be accessible by telephone.
9. If the subject is a female of childbearing age, she must have had: a hysterectomy, or tubal ligation, or otherwise be incapable or pregnancy, or have practiced one of the following methods of contraception for at least one month prior to study entry: hormonal contraceptives, spermicide and barrier, intrauterine device, partner sterility.
10. Female of childbearing age must have had a negative urine pregnancy test within one week of study entry. 11. Prior to participation in this study, each subject must sign an informed consent.

Exclusion Criteria:

1. Subjects who do not meet inclusion criteria.
2. Subjects who are not able to abstain from alcohol for 24 hours prior to each evaluation.
3. Subjects who can not maintain an identical dose of any medicine that may affect PD symptoms or signs during their entire study involvement.
4. Subjects who have exhibited meaningful psychiatric disease not thought to be related to PD. (Depression and psychosis typical for PD will not be excluded). 5. Subjects who have previously taken memantine.

6. Subjects currently taking Amantadine. 7. Subjects with greater than moderate dementia (MMSE<24). 8. Subjects with co-morbid disease that in the investigators decision could interfere with treatment with memantine.