The purpose of the study is to assess the efficacy and safety of XP21279/Carbidopa in comparison to Sinemet as well as evaluate the pharmacokinetics (PK) of levodopa after administration of XP21279/Carbidopa and Sinemet and to explore exposure-response relationships in a subset of subjects.

Inclusion Criteria:

1. Subjects must have predictable motor fluctuations of the wearing off type, defined by meeting the following criteria based on the on/off diaries recorded over 3 days in the Screening Period:

   • Wearing-off in at least half (50%) of inter-dose intervals between the first and the last daily doses averaged over the 3 diary days, and
   • An average daily "off" time of 2 hours after the first "on" of the day through awake time up to midnight.

2. Subjects must be on one of the following stable QID or 5 times daily regimens for at least 4 weeks prior to Screening: Sinemet® or carbidopa-levodopa, with a total daily dose ranging from 400 mg to 1000 mg of levodopa

Exclusion Criteria:

1. History, signs, or symptoms suggesting the diagnosis of secondary or atypical Parkinsonism.
2. Subject has moderately or severely disabling dyskinesias for greater than 25% of the waking day
3. Subjects who have significant neurological symptoms not accounted for by Parkinson's disease
4. Subjects who are taking Sinemet® CR, Parcopa®, concomitant COMT inhibitors (i.e., entacapone or tolcapone), Stalevo®, or benserazide containing levodopa preparations Madopar® or Prolopa®.