The objective of this study is to evaluate the acute effects of a selective agonist of serotonin 5-HT1A receptors on the severity of parkinsonian signs and levodopa-associated motor response complications in patients with moderately advanced Parkinson's disease. In a controlled proof-of-principle clinical trial, the efficacy of EMD 128130 will be assessed through the use of validated motor function scales. Safety will be monitored by means of frequent clinical evaluations and laboratory tests.