Within 3 months of the dose-finding phase, treatment will begin. Patients will receive seven doses of JD-1730 or placebo (an inactive substance) via puffs from an oral spray together with levodopa infusions over a 3-week period. The doses are given on days 1, 2, and 3 of the first week and then approximately twice a week for the next 2 weeks. For these doses, patients are hospitalized 4 days the first week and 2 days each for the next 2 weeks. All participants will receive placebo at some time during the study, and a few patients, selected at random, will receive only placebo the entire 3 weeks. The procedure for the infusions is the same as that for the dose-finding phase, with frequent evaluation of symptoms. Also, small blood samples are drawn up to three times each study day. At the end of the third week, patients will be discharged from the hospital. Their anti-parkinsonian medications may be readjusted, as needed. Patients will be contacted 2 weeks after the end of the study for a check on side effects and, if necessary, will be scheduled for a follow-up evaluation at the clinic.