L-Dopa is rapidly absorbed after oral administration, and peak plasma levels are reached after 30 to 120 minutes. The half-life of L-Dopa is 90 minutes. Absorption of L-Dopa can be significantly reduced by changes in gastric pH and by ingestion with meals. Bromocriptine and ropinirole are rapidly absorbed but undergo first-pass metabolism such that only about 30 to 55% of the dose is bioavailable. Peak concentrations are achieved 1.5 to 3 hours after oral administration. The half-life of ropinirole is 6 hours. Pramipexole is rapidly absorbed with little first-pass metabolism and reaches peak concentrations in 2 hours. Its half-life is 8 hours. Oral forms of apomorphine have been studied but are not available in the United States. Subcutaneous
apomorphine injection results in rapid and controlled systemic delivery, with linear pharmacokinetics over a dose ranging from 2 to 8 mg.

After L-Dopa enters the dopaminergic neurons of the central nervous system (CNS), it is converted into the neurotransmitter dopamine. Apomorphine, bromocriptine, ropinirole, and pramipexole act directly on dopamine receptors. Dopamine, pramipexole, and ropinirole bind about 20 times more selectively to dopamine D₃ than D₂ receptors; the corresponding ratio for bromocriptine is less than 2:1. Apomorphine binds selectively to D₁ and D₂ receptors, with little affinity for D₃ and D₄ receptors. L-Dopa, pramipexole, and ropinirole have no significant activity at nondopaminergic receptors, but bromocriptine binds to serotonin 5-HT₁ and 5-HT₂ and α₁-, α₂-, and β-adrenergic receptors.