It is now well-established that there are three families of opioid peptide precursors, and endogenous opioid peptides exhibit relatively low pharmacologic selectivity for opioid receptor types (μ, δ, κ) and are metabolically unstable. While peptide ligands for δ opioid receptors (including δ1 subtype-selective peptides like DPDPE) exist, no selective nonpeptide δ1 agonists have been reported. Here we report the design and pharmacologic evaluation of the first selective nonpeptide δ1 opioid agonist, 7-spiroindanyloxymorphone (SIOM). The design was based on structure-activity relationship studies of the prototypical δ opioid antagonist naltrindole (NTI): we synthesized SIOM, whose address moiety has a conformation similar to the Phe4 phenyl group of DPDPE (as determined by NMR and energy-minimization studies), by modifying oxymorphone via 3-O-benzylation, alkylation with α,α'-dibromo-o-xylene, and hydrogenolysis. Pharmacologic evaluation showed: In vitro, SIOM was a full agonist in mouse vas deferens (IC50 = 19 nM) and a partial agonist in guinea pig ileum; naltrindole strongly antagonized its effect (Ki = 1.5 nM) while naloxone was feeble, confirming δ receptor mediation. Binding assays revealed SIOM’s highest affinity was for δ1 receptors (order: δ1 > δ2 > μ >> κ), supporting δ1 selectivity. In vivo antinociceptive testing (mouse tail-flick) showed SIOM was nearly 7 times more potent than DPDPE on icv administration, active on sc administration, and the δ1 antagonist 7-benzylidenenaltrexone (BNTX) was more effective than other antagonists in blocking its effect. At a low dose (0.5 nmol icv) that did not produce antinociception, SIOM antagonized the agonist effect of DPDPE and morphine but not DSLET (δ2) or U50488 (κ), indicating it acts as a δ1 antagonist at low doses and a δ1 agonist at higher doses, and antagonizes μ receptors without affecting δ2 or κ. A possible explanation is that δ1 receptors have two recognition sites: SIOM binds the antagonist site at low concentrations and the agonist site at high concentrations, with allosteric coupling between the sites. These data suggest SIOM’s agonist activity involves pharmacophoric conformations common with δ1-selective peptides when bound to the δ1 agonist recognition site, and the coplanarity of the aromatic group in naltrindole analogs may affect binding to agonist/antagonist sites.