• Mean change from Baseline to endpoint of 12 weeks of treatment in the 3-day average (based on subject diaries) of time in hours per day spent in the "off" state [ Time Frame: 12-week double-blind treatment period ] [ Designated as safety issue: Yes ]
  

• Mean change from Baseline in "off" time per day; "on" time per day (no dyskinesias; with and w/o troublesome dyskinesias); awake time per day in the "on" state; absolute time of dyskinesias, total sleep time, frequency of sleep attacks, and UPDRS [ Time Frame: 12-week double-blind treatment period ] [ Designated as safety issue: Yes ]
  

Inclusion Criteria:

• Subjects must be 30 years of age, of either sex and of any race, with adiagnosis of moderate to severe idiopathic Parkinson's disease for at least 5 years.
• Women of childbearing potential must have a negative serum pregnancy test at Visit 2. If subject is postmenopausal (not surgically induced), she must be postmenopausal by history for at least 2 years before study entry. If not, proper birth control must be used.

Note: Acceptable methods of birth control include oral or injectable hormonal contraceptive, medically prescribed IUD, and double-barrier method (eg, condom in combination with spermicide). Bilateral tubal ligation is an acceptable method of birth control for this study.

• Subjects' clinical laboratory tests (CBC, blood chemistries, and urinalysis) must be within normal limits or clinically acceptable to the investigator/sponsor.

Exclusion Criteria:

• Subjects with any form of drug-induced or atypical parkinsonism, cognitive impairment (Mini-Mental State Examination [MMSE] score <=23), a history of DSM IV diagnosed major depression, unstable mild depression or psychosis, or subjects taking tolcapone will be excluded. (Subjects with mild depression who are well controlled on a stable dose of an antidepressant medication for at least 4 weeks before screening will be eligible.)
• All subjects with a severe or ongoing unstable medical condition will be excluded including those with a history of poorly controlled diabetes, obesity associated with metabolic syndrome, uncontrolled hypertension, cerebrovascular disease, or any form of clinically significant cardiac disease, symptomatic orthostatic hypotension, renal failure, history of abnormal renal function, seizures, alcohol/drug dependence, or previous surgery for Parkinson's disease.
• Average daily consumption of more than two 4-oz (120 mL) glasses of wine or their equivalent.
• Because it is not known whether SCH 420814 passes into breast milk and because the effects, if any, of SCH 420814 on the developing human are unknown, women who are breastfeeding or who are considering breastfeeding are excluded from this trial.
• Subjects with allergy/sensitivity to study drug or its excipients.
• Subjects with any clinically significant condition or situation, other than the condition being studied that, in the opinion of the investigator, would interfere with the study evaluations or optimal participation in the study.
• Subjects who have used any other investigational drugs within 30 days of Screening.
• Subjects who are participating in any other clinical study.