The endogenous opioid heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF) has been shown to interact with multiple opioid as well as non-opioid sites in mammalian brain membranes. To increase the stability and bioavailability of MERF, new synthetic derivatives with D-amino acid substitutions were prepared and studied. One of the new compounds in this series, Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN), had only moderate affinity in competing with [3H]MERF, whereas it displayed the highest potency in producing antinociception following intrathecal administration. DADN was radiolabeled with 41Ci/mmol specific activity. Specific binding of [3H]DADN was saturable, stereoselective and of high affinity. Chemical stability, increased micro-receptor selectivity, and hydrophobicity of the peptide all contribute to the effectiveness observed in biochemical and pharmacological studies.