A series of deltorphin (DLT: Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) analogs in which Leu5 and/or Met6 were mainly replaced by t-Leu(Tle) and/or N alpha-alkylated glycine were synthesized and examined for their receptor binding properties and in vitro bioactivities by guinea pig ileum (GPI) and mouse vas deferens (MVD) assays. [Tle5]DLT(2) showed a dramatic decrease in the MVD potency when compared to the parent peptide and was found to have a potent delta receptor antagonist activity against various delta agonists with Ke values of 16-311 nM. Interestingly, the antagonist potency of 2 against DPDPE as agonist was 20-fold weaker than that against deltorphins or Leu-enkephalin. Among the analogs in which Met6 was replaced by an N alpha-alkylated Gly residue, [N alpha-isobutyl-Gly6]DLT(5) behaved as a mixed mu antagonist/delta agonist while its isomeric analogs in which the N alpha-alkyl is n-butyl (4) or (R or S) sec-butyl (6a,b) were very potent delta receptor agonists. Analogs 2, 4, 6a, and 6b were highly stable against rat brain and rat plasma enzymes and thus may represent a starting point for the development of novel receptor-specific compounds useful as ligands for studies of opioid receptors.