Novel Leu-enkephalin analogs 10a–c in which the Tyr1, Gly2, or Gly3 residue of Leu-enkephalin 9 was replaced with α-amino squaric acidanalog Sq-Tyr 8b or Sq-Gly 8a were synthesized starting from 14 or 18. Conformational analysis of 10a–c together with its model compound 26 and their opioid receptorbinding activity were evaluated.