Inclusion Criteria:

1. Male and female patients 40 - 80 years of age with a diagnosis of idiopathic PD < 10 years. The diagnosis of idiopathic PD will be based on the medical history, neurologic examination, current response to anti-PD medication(s), and the presence of at least two of the following at the time of diagnosis: resting tremor, bradykinesia, or rigidity.
2. Females must be postmenopausal for >= 12 months, surgically sterile, or agree to use acceptable forms of contraception. A negative serum pregnancy test must be confirmed prior to first dose for women of childbearing potential.
3. Clinical diagnosis of idiopathic mild to moderate PD characterized by a Hoehn and Yahr rating of 1 to 3 in the 'Off' state (measured before the first dose of anti-PD medications on the day of assessment).
4. UPDRS Motor 'Off' rating of 8-30 (measured before the first dose of anti-PD medications on the day of assessment).
5. Mini-Mental Status Examination (MMSE) score of <= 25.
6. Currently treated with an optimized dose of a dopamine agonist (stable dose for >= 1 month prior to randomization and treatment is optimized in the opinion of the Investigator).
7. In the judgment of the Investigator the patient will not require L-Dopa therapy within the 3 months after randomization.
8. Concomitant therapy with amantadine, selegiline, or anticholinergics is permitted, but not required. If the patient is treated with any of these medications the dose of this medication must be judged optimal and stable for > 1 month prior to randomization.

Exclusion Criteria:

1. Presence of motor fluctuations including drug-induced dyskinesia, but excluding the pre-dose 'Off' state (prior to the first dose of anti-parkinsonian medication(s) on the day of assessment).
2. History of surgical treatment of PD.
3. Presence of clinical signs consistent with a neurologic disorder other than PD including, but not limited to, progressive supranuclear palsy, multiple system atrophy (Shy-Drager syndrome, olivopontocerebellar degeneration, striatonigral degeneration), corticobasal degeneration, Pick's disease, diffuse Lewy body disease, dementia, schizophrenia, psychosis, or hallucinations.
4. Presence of clinically significant depression as measured by the HAM-D Scale with a score > 16. If the patient is on an antidepressant, the dose must be judged optimal and stable for ³ 1 month prior to randomization.
5. Presence of clinically significant, in the judgment of the Investigator, urinary incontinence, cardiac arrhythmia, or symptomatic orthostatic hypotension.
6. History of seizure disorder or the occurrence of 1 or more seizures within 1 year before screening.
7. Any medical disability (e.g., peptic ulcer disease, severe degenerative arthritis, compromised nutritional state) or laboratory abnormality (e.g., serum creatinine > 2.0 mg/dL) that may interfere with the protocol-specified safety and efficacy measurements, present an unacceptable risk to the patient's well-being, or compromise the patient's ability to provide informed consent.
8. Recent history, within the 2 years before screening, of drug or alcohol abuse.
9. History of anaphylaxis.
10. Previous treatment with L-Dopa for > 90 days or treatment with L-Dopa within 30 days prior to the baseline assessment.
11. Treatment within the 3 months before the Baseline SPECT Scan with modafinil.
12. Treatment within the 6 months before screening with neuroleptics, methylphenidate, metoclopramide, cinnarizine, flunarizine, reserpine, alpha methyldopa, amphetamine, or monoamine oxidase-A (MOA-A) inhibitors.
13. Previous exposure to GPI 1485 (previously AMG-474-00).
14. Treatment with an investigational agent within the 30 days before screening or scheduled to receive an investigational agent other than that specified by this protocol during the course of this study.
15. Females that are pregnant, breast feeding, or do not agree to use an acceptable form of contraception.