The N-alkyl analogues (N-ethyl through N-heptyl), branched N-alkyl chain analogues (N-isopropyl, N-2-methylpropyl, and N-3-methylbutyl), and N-alkenyl analogues ((E)-N-3-methylallyl (crotyl), N-2-methylallyl, and N-3,3-dimethylallyl) were prepared in the noroxymorphindole series (17-substituted-6,7-dehydro-4,5alpha-epoxy-3,14-dihydroxy-6,7:2',3'-indolomorphinans), and the effect of the N-substituent on opioid receptor affinity, selectivity, and efficacy was examined using receptor binding assays, [(35)S]GTPgammaS efficacy determinations, and smooth muscle functional assays (electrically stimulated mouse vas deferens and guinea pig ileum). All of the compounds acted as opioid antagonists, including those with N-substituents which usually confer either weak agonist-antagonist behavior (N-ethyl) or potent opioid agonist activity (N-pentyl) in morphinan-like ligands which interact with the mu-receptor. Several N-substituted noroxymorphindoles were found to be more mu/delta-selective than naltrindole (NTI). The N-2-methylallylnoroxymorphindole, in particular, was found to be more selective than NTI in receptor binding assays (mu/delta = 1700 vs 120; kappa/delta = 810 vs 140), as an antagonist in the GTPgammaS assay (mu/delta = 170 vs 140; kappa/delta = 620 vs 160), and considerably more selective than NTI in the functional assays (mu/delta > 2200 vs 90). It also had high affinity for the delta-opioid receptor (K(i) = 4.7 nM in the binding assay) and high antagonist potency (1.2 nM in the GTPgammaS assay; 8.9 nM in the MVD assay).