11C-NOP-1A is a new radioligand for thenociceptin/orphanin FQ peptide (NOP) receptor, with high affinity (Ki, 0.15 nM) and appropriate lipophilicity (measured logD, 3.4) for PET brain imaging. 11C-NOP-1A is the the first successful radioligand to image NOP receptors in rat and monkey brain. 11C-NOP-1A is a selective antagonist at the NOP receptor and has high affinity and appropriate lipophilicity for blood–brain barrier permeability. 11C-NOP-1A imaging in rhesus monkeys showed high brain uptake and a large receptor-specific signal and could be quantified with the gold standard method of compartmental modeling. In humans 1C-NOP-1A reliably quantified NOP receptors in human brain both in large brain regions and at a voxelwise level using parametric imaging. The radiation absorbed dose in humans was similar to that observed with other 11C-labeled ligands and would allow multiple scans of a single subject. Thus, 11C-NOP-1A is a promising radioligand for reliably quantifying NOP receptors in human brain.