A cyclic Leu-enkephalin mimetic containing a 7-membered ring, and two linear analogues, has been prepared on solid phase. In the cyclic mimetic the intramolecular (1-4) hydrogen bond found in crystalline Leu-enkephalin has been replaced by an ethylene bridge. In addition, the amide bond between Tyr1 and Gly2 has been replaced by a methylene ether isostere and Gly3 has been deleted. The two linear analogues both contain the methylene ether isostere instead of the Tyr1-Gly2 amide bond and the shorter of the two lacks Gly3. The three compounds, and a beta-turn mimetic analogous to the 7-membered turn mimetic but with Gly3 included, were evaluated for specific binding to micro- and delta-opioid receptors in rat brain membranes. With the exception of the beta-turn mimetic the three other Leu-enkephalin analogues all bound with varying affinity to the micro- and delta-opioid receptors. From the results it could be concluded that Leu-enkephalin binds in a turn conformation to the opiate receptors, but that this conformation is not a (1-4) beta-turn.