Parkinson's disease (PD) is a neurological condition, which means it affects the brain. This study will evaluate how ABBV-951 is absorbed under the skin of participants with PD when administered to arm, thigh and flank compared to the abdomen.

ABBV-951 is an investigational drug being developed for the treatment of PD. Study doctors randomly assign participants to 1 of 4 groups, called treatment arms. Each treatment arm receives ABBV-951 administered in a different order in the arm, high, flank and abdomen. Approximately 12 adult participants over 30 years with a diagnosis of PD will be enrolled in approximately 10 sites in the United States.

Participants will receive continuous (24hours/day) subcutaneous infusion of ABBV-951 for 2 consecutive days for each infusion site (arm, thigh, flank and abdomen), for a total duration of treatment up to 12 days.

There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will be confined at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires. Adverse events will be monitored throughout the study.

Inclusion Criteria:

• Diagnosis of idiopathic Parkinson's disease (PD) that is Levodopa-responsive.
• Must be taking a minimum of 400 mg/day of levodopa equivalents (LE) and be judged by the investigator to have motor symptoms inadequately controlled by current therapy.
• Must have recognizable/identifiable "Off" and "On" states (motor fluctuations), and have a minimum daily average of 2.5 hours of "Off" time (with a minimum of 2 hours of "Off" time each day).

Exclusion Criteria:

- History of significant skin conditions or disorders (e.g., psoriasis, atopic dermatitis, etc.) or evidence of recent sunburn, acne, scar tissue, tattoo, open wound, branding, or colorations that in the investigator's opinion would interfere with the infusion of the study drug or could interfere with study assessments.