The N-benzyl group of N-benzylnaltrindole (1, BNTI), a potent and selective delta 2 opioid receptor antagonist, was employed as a scaffold to hold electrophilic moieties (isothiocyanate and haloacetamide) in an effort to obtain selective affinity labels (2-4 and 8-11). The corresponding acetamide derivatives (5-7) also were synthesized to serve as nonelectrophilic controls. The o- and p-isothiocyanates (2 and 4) and the haloamides (8-11) were selective delta opioid receptor antagonists in the mouse vas deferens (MVD) preparations, while the meta isomer 3 was a delta-selective full agonist (IC50 = 5 nM). The fact that the effect of 2 and 4 was found to increase as a function of time in MVD suggests a covalent mechanism for the wash resistant component. The m-isothiocyanate 3 was found to be a delta-selective and irreversible agonist in the MVD, and it is suggested that it may be covalently binding to an agonist recognition site. In the mouse abdominal stretch antinociceptive assay, compounds 2-4 and 9 were delta-selective antagonists but exhibited delta 2/delta 1 selectivity ratios than that of BNTI.