The properties of two prospective positron emission tomography (PET) ligands for the delta-opioid receptor, N1'-fluoroethyl-naltrindole (BU97001) and N1'-fluoroethyl-(14-formylamino)-naltrindole (BU97018) were investigated. Both were antagonists in the mouse vas deferens, and showed high affinity and selectivity, 1.81 nM and 3.09 nM respectively. [3H]BU97001 binding to rat whole brain was also of high affinity, K(D) of 0.42 nM of and B(MAX) of 59.95 fmol mg of protein(-1). In autoradiographic studies, it was found to bind to brain areas previously shown to be associated with the delta-opioid receptor and good correlations were found to exist with naltrindole and DPDPE. BU97018 and especially BU97001 appear to show good potential as delta-opioid receptor PET ligands with the incorporation of 18F.