When these dose rates are determined, patients will begin treatment in one of two groups. One will take LY300164 3 times a day, along with levodopa, for 3 weeks. The second group will take placebo tablets (a look-alike tablet with no active ingredient) and levodopa on the same schedule as the LY300164 group. A brief medical examination and routine blood and urine tests will be done weekly. The drug dose will be increased every 3 to 4 days until significant side effects occur or the maximal dose is reached. Patients will be closely monitored for 4 hours after every increase. At the end of the 3 weeks, or when the maximal dose is reached, patients will be readmitted to the hospital for 2 to 3 days for a second levodopa dose-finding study, while continuing on LY300164 or placebo. After this test, patients will resume taking levodopa and the experimental drug or placebo as before for another 2 weeks.