The regional distribution of N1'-([11C]methyl)naltrindole ([11C]MeNTI) in vivo in mouse brain correlates with established delta opioid receptor densities in vitro. [11C]MeNTI binding is blocked by naltrindole, a delta antagonist, but not by cyprodime, a mu antagonist, of by (+/-)-U50,488, a kappa agonist. Thus, [11C]MeNTI selectively labels central delta opioid receptors in vivo in mouse, and shows promise for positron emission tomography studies of delta sites in human brain.