Expression and release of nociceptin/orphanin FQ (N/OFQ) are elevated in the substantia nigra reticulata of 6-hydroxydopamine-hemilesioned rats, suggesting a pathogenic role for N/OFQ in Parkinson's disease. In this study, we investigated whether elevation of N/OFQ expression in 6-hydroxydopamine-hemilesioned rats selectively occurs in substantia nigra and whether hypomotility following acute haloperidol administration is accompanied by a rise in nigral N/OFQ levels. Moreover, to prove a link between N/OFQ and idiopathic Parkinson's disease in humans, we measured N/OFQ levels in the cerebrospinal fluid of parkinsonian patients undergoing surgery for deep brain stimulation. In situ hybridization demonstrated that dopamine depletion was associated with increase of N/OFQ expression in substantia nigra (compacta +160%, reticulata +105%) and subthalamic nucleus (+45%), as well as reduction in caudate putamen (-20%). No change was observed in globus pallidus, nucleus accumbens, thalamus, and motor cortex. Microdialysis coupled to the bar test allowed to demonstrate that acute administration of haloperidol (0.8 and 3 mg/kg) increased nigral N/OFQ levels (maximally of +47% and +53%, respectively) in parallel with akinesia. A correlation with preclinical studies was found by analyzing N/OFQ levels in humans. Indeed, N/OFQ levels were found to be approximately 3.5-fold elevated in the cerebrospinal fluid of parkinsonian patients (148 fmol/ml) compared with nonparkinsonian neurologic controls (41 fmol/ml). These data represent the first clinical evidence linking N/OFQ to idiopathic Parkinson's disease in humans. They strengthen the pathogenic role of N/OFQ in the modulation of parkinsonism across species and provide a rationale for developing N/OFQ receptor antagonists as antiparkinsonian drugs.